bundle: warn when a task parameter is an empty string under the terraform engine

[ankit481]

Changes

Adds a validate-time warning when a task's parameters list contains an empty string element and the bundle deploys with the terraform engine. Such deploys currently pass bundle validate and then fail while creating the job with a provider panic:

Error: cannot create job: panic: task: spark_python_task: parameters: task.0.spark_python_task.0.parameters[<nil>] is not a string

The warning names the config location of the empty element and suggests removing it or deploying with the direct engine, which handles the same configuration correctly. It is emitted only when the effective engine is not direct, following the same engine resolution as validate.TFOnlyReferences.

This is a draft on purpose. The underlying defect looks like it sits in the terraform provider's schema read layer (the generated bundle.tf.json is well formed and renders the element as ""), so the real fix may belong in terraform-provider-databricks. This PR covers the CLI side: catching it at validate time instead of letting terraform apply crash. Direction welcome on warning versus error, the exact wording, and whether you would rather see only the regression tests merged here.

Related: #5950. The warning is a guard on the CLI side; the provider panic itself remains open.

Why

An empty parameter usually comes from a bundle variable that is deliberately blank in some targets, for example a per-developer prefix that is empty in shared environments. That configuration is legal, validate passes, and the crash appears only at deploy time in whichever target leaves the variable empty. We hit this four times over two months before spotting the pattern. A warning at validate time with the file and line turns an opaque provider panic into something the user can act on before deploying.

Tests

• Unit tests for the new validator: warnings for spark_python_task, python_wheel_task, and a for_each_task nested spark_jar_task under the terraform engine, and silence under the direct engine or when no parameter is empty.
• Acceptance test bundle/variables/empty-in-task-params runs validate and deploy under both engines. The direct variant deploys successfully and jobs/create receives the empty string. The terraform variant records the new warning and the current provider panic, and also records the parameters as rendered into bundle.tf.json to show the generated configuration is well formed.
• Acceptance test bundle/variables/empty-in-task-params-variants pins that every empty-variable shape (short, long, and null target overrides, BUNDLE_VAR_*, --var, a variable referencing another variable, and a complex variable field) resolves to an empty string rather than null.

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• PR number: 5951
• Commit SHA: b7d28b8eb3e3f75440b53c737926d033c11af1ee

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