improve semantics for rule_priority

[skalexch]

Summary

Clarify the wording of rule_priority in order to:

1. Make sure to differentiate the existence of the column from the existence of values.
2. Make sure to only interpret an empty rule_priority value as 0 only when the field exists in the header.

Describe the Problem

Elaborated on further in google#575, the semantics of rule_priority can be vague and might cause interpretation issues. The existence of the field affects matching fare leg predicates.
Therefore, any misinterpretation of the existence of the field vs the value affects the interpretation of an empty predicate (e.g. an empty network_id is interpreted differently based on the existence of the COLUMN rule_priority.

Use Cases

Proposed Solution

Clarify the spec by:

1. Changing "the rule_priority field exists" to "the rule_priority field is defined in the table header", and doing the same for the negative version of that sentence.
2. Changing "An empty value for rule_priority is treated as zero." to "If the rule_priority field is defined in the header, an empty value for rule_priority is treated as zero."

Type of change

GTFS Schedule

• Functional Change
• Non-Functional Change
• Documentation Maintenance

GTFS Realtime

• Specification Change
• Specification Change (Experimental Field)

Additional Information

Proposed Discussion Period

Testing Details

• Consumer(s):
• Producer(s):
• Estimated Testing Period:

Proposal Update Tracker

Date	Update Description
(YYYY-MM-DD)	(Brief description of the update)

Checklist

• I have read and understood the GTFS Change process.
• I have read and understood the GTFS Guiding Principles.
• I understand the Role of Advocate and the Responsibilities attached to it.
• I have signed the Contributor License Agreement (CLA).
• I have ensured that this proposal is not covered by an existing Pull Request.