Skip notifier templating with empty template_fields

[DaveT1991]

Summary

Skip notifier templating setup when a notifier does not define any template fields.

When airflow dags test executes DAG callbacks, the callback context can include a SerializedDAG. BaseNotifier.render_template_fields() currently always requests a template environment from the context DAG, even when there is nothing to render. For notifiers that rely on the default empty template_fields, that raises AttributeError before the callback can run.

This change returns early when template_fields is empty and adds a regression test that exercises a notifier callback with a serialized DAG context.

Closes #64649

Testing

• Added a regression test in task-sdk/tests/task_sdk/bases/test_notifier.py
• git diff --check

[boring-cyborg]

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[DaveT1991]

Any updates about my code submission?

[Copilot]

Pull request overview

Fixes notifier callbacks failing under airflow dags test when the callback context contains a SerializedDAG by avoiding unnecessary Jinja environment setup when there are no template fields to render.

Changes:

• Return early from BaseNotifier.render_template_fields() when template_fields is empty.
• Avoid calling dag.get_template_env() when the context DAG object doesn’t support it (e.g., SerializedDAG), falling back to a default environment.
• Add regression tests covering notifier invocation and template rendering with a serialized DAG in the context.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
task-sdk/src/airflow/sdk/bases/notifier.py	Skips templating work when template_fields is empty; adds safer template-env selection for non-DAG context objects.
task-sdk/tests/task_sdk/bases/test_notifier.py	Adds regression tests for notifier callback execution and template rendering when context["dag"] is a SerializedDAG.