Ensure script variables fall back to their defined defaults when a value is not
submitted (e.g. via "Run again" or other minimal POSTs).
- Populate omitted script variables with their initial/default values before
validation and job enqueueing
- Treat falsy defaults (e.g. False/0) as valid defaults
- Add a test asserting defaults are included in enqueued job data
- Remove the redundant default from ScriptValidationErrorTest
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