fix(task-sdk): respect default parameter in xcom_pull when map_indexes is not set

[armorbreak001]

Summary

In RuntimeTaskInstance.xcom_pull(), when map_indexes is not specified (the default NOTSET path), the code always appends None to the result list when no XCom is found, ignoring the caller-provided default parameter.

The explicit map_indexes branch (below) correctly uses default. This fix aligns the non-map_indexes path with that behavior.

Before: ti.xcom_pull(task_ids="t", key="k", default="fallback") returns None when no XCom exists.
After: Returns "fallback" as expected per the docstring.

Fixes #64295

[boring-cyborg]

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