Give the funnel functions the query's null handling option

[Jackie-Jiang]

Summary

Part of #19218. Follows #19316 (sketch-backed distinct counts) and #19326 (value aggregations).

The six funnel types — FUNNELMAXSTEP, FUNNELCOMPLETECOUNT, FUNNELSTEPDURATIONSTATS, FUNNELMATCHSTEP,
FUNNELEVENTSFUNCTIONEVAL and FUNNELCOUNT — never received the query's null handling option, so a null row was
read as the column default whatever the query asked for. Both funnel architectures are covered:
FunnelBaseAggregationFunction and its four subclasses, the standalone FunnelEventsFunctionEval, and
FUNNELCOUNT, whose aggregation lives in AggregationStrategy and its three strategy subclasses.

Which null disqualifies a row is decided per column

A funnel reads more than one column, so it cannot extend NullableSingleInputAggregationFunction. It follows the
multi-input precedent set by CovarianceAggregationFunction — holding the flag itself and skipping rows through a
local helper — and each column's rule follows from what that column is for:

Column	Rule	Why
Timestamp (window funnels)	skip the row	an aggregate ignores rows whose input is NULL, and the event has no position in the window
correlateBy key (FUNNELCOUNT)	skip the row	the key is what distinct counts are taken over; with a composite key a null in any component leaves the whole key undefined
Step predicates	not gated	a predicate over NULL is UNKNOWN, which SQL treats as not satisfied wherever a boolean is consumed, so a null step already means that step did not match
Extra fields	not gated	payload on a matched event, not an input; dropping the row would lose an event that really happened

There is no SQL standard for funnel functions themselves, but each column role has one, and this is where they land.

Both were real defects

BaseTransformFunction#getNullBitmap ORs its arguments' bitmaps, so null timestamps genuinely arrive. One was
being read as the column default and planted a step event at a fabricated time, corrupting window ordering and
able to manufacture funnel completions. A null correlateBy key was read as the default's dictionary id — a real
user's id — so the funnel credited that user with a step they never took.

Testing

FunnelNullHandlingTest is new and covers both architectures, including that KEEP_ALL must not fabricate a dummy
event for a null-timestamp row. Every case was verified by mutation rather than by inspection.

SyntheticBlockValSets gains a single-value dictionary-ids fixture, and its class comment now records that the
null bitmap is independent of the values — which is why a test wanting "nothing was aggregated" in the disabled
mode needs an untouched holder rather than an all-null block.

AggregationFunctionNullContractTest gains multi-value block shapes, which make SUMARRAYLONG and SUMARRAYDOUBLE
drivable by the generic harness for the first time — the set of undrivable functions shrinking rather than growing —
and a pin on the funnels' empty answer, which is zero rather than NULL and was previously unpinned.

Contract documentation

With this the option reaches every user-facing aggregation, so the first known deviation on AggregationFunction
is removed. It is replaced by what the series settled: how a multi-input function decides which nulls matter, and
the three functions that stay outside the contract (ParentExprMinMax, ChildExprMinMax, TimeSeriesAggregation
— none of them an aggregate a query can name). The multi-stage engine's unconditional null handling remains as the
sole outstanding deviation.

Known, deliberately not fixed here

FunnelEventsFunctionEval#getExtraFieldsBlocks and TimeSeriesAggregationFunction switch on the logical value
type rather than the stored type, so BOOLEAN and JSON columns are rejected where they would work. That is a
type-dispatch bug rather than a null-handling one, and it widens accepted input, so it is going in its own PR.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 47.78157% with 153 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.03%. Comparing base (3c92923) to head (e56d88c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...w/FunnelEventsFunctionEvalAggregationFunction.java	6.34%	59 Missing ⚠️
...egation/function/HistogramAggregationFunction.java	46.98%	42 Missing and 2 partials ⚠️
...gregation/function/funnel/AggregationStrategy.java	42.25%	38 Missing and 3 partials ⚠️
...n/funnel/window/FunnelBaseAggregationFunction.java	84.48%	2 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #19332      +/-   ##
============================================
+ Coverage     66.97%   67.03%   +0.05%     
  Complexity     1424     1424              
============================================
  Files          3463     3463              
  Lines        222078   222149      +71     
  Branches      34957    34954       -3     
============================================
+ Hits         148743   148914     +171     
+ Misses        61467    61352     -115     
- Partials      11868    11883      +15     

Flag	Coverage Δ
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	?
java-25	67.03% <47.78%> (+0.05%)	⬆️
lane-a	100.00% <ø> (ø)
lane-b	0.00% <ø> (ø)
temurin	67.03% <47.78%> (+0.05%)	⬆️
unittests	67.03% <47.78%> (+0.05%)	⬆️
unittests1	57.77% <47.78%> (+0.08%)	⬆️
unittests2	38.86% <0.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.