Add nullptr check before strcmp to satisfy GCC's -Werror=nonnull
warning. The expected_strip parameter can be nullptr in some test
cases, so we need to check for null before passing to strcmp.

This fixes the Fedora 41 gcc release build failure.

When expected_strip is nullptr, the test expects that the paramName
value should be empty (i.e., no parameter was extracted from the URI).
Add a check to verify uri_strip is indeed empty in this case.

Remove early cjose_jws_release calls that caused double-free when
validation failed. The function already releases jws unconditionally
at the end, so the early releases on the error paths resulted in
freeing the same pointer twice.

This was a pre-existing bug in the strcmp path and was replicated
in the new nullptr check path added in the previous commit.