Port skipif.inc files to EXTENSIONS

Declare tentative return types for ext/session (#7005)

Fix #80889: Cannot set save handler when save_handler is invalid

There is no need to require a (valid) save_handler to be set, when a
user handler is supposed to be set. We just have to

Fix #80889: Cannot set save handler when save_handler is invalid

There is no need to require a (valid) save_handler to be set, when a
user handler is supposed to be set. We just have to make sure, that
no user handler is already set in this case.

Closes GH-6788.

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