[1.9.x] Fixed #25840 -- Fixed BaseCache.get_or_set() on the DummyCache backend.

This also fixes a possible data eviction race condition between
setting and getting a key. Another thread could remove the key
before get_and_set() accesses it again. In this case, now the
default value will be returned instead of None.

Backport of 8e838d9c from master