Initialise pam response in all converse conditions

Merged David Edmundson requested to merge work/davidedmundson/crash_fix into master

This turned out to be an issue for pam_python when sending an info/error message that shouldn't have a response some code still tried to free the result of any prompt. Somewhat questionable, but we should still be returning a valid struct full of nulls.

BUG: 455608

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