Bug fix for ExternalTaskSensor with failed_states, allowed_states, and execution_date_fn set

[eitanme]

I believe this PR fixes #16204 where the ExternalTaskSensor would hang indefinitely when an execution_date_fn is used, failed_states/allowed_states are set, and external DAGs have mixed states upstream.

Apologies for not having a better handle on the unit tests to add coverage here, I don't have a good handle on the structure for this case and lack the time to dig deeper. Given the nature of the bug, I hope this PR is still useful.

[boring-cyborg]

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