fix: prevent early return in check_for_write_conflict loop

[Kundan-CR7]

Fixes a logic issue in Variable.check_for_write_conflict where a return None
statement was placed inside the loop iterating over secrets backends.

Because of this, the function exited after the first iteration, preventing
remaining backends from being checked. This could lead to missed conflict detection
in environments with multiple secrets backends.

The fix moves the return None statement outside the loop so that all backends
are evaluated before returning.

This change corrects the control flow without introducing any new behavior.

closes: #64060

[boring-cyborg]

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[SameerMesiah97]

Looks fine. It seems like this codepath is not currently covered by tests so I think you should add one. A simple case where the first backend returns None and a later one returns a value should be enough.

CI needs to be triggered.

[Kundan-CR7]

@SameerMesiah97 Thanks for the suggestion! Added a test to cover multiple backends where the first returns None and a later one returns a value, ensuring all backends are checked.