cache-adaptor: Fix crash when no toolkit name is set
atk_get_toolkit_name may return NULL when e.g. running the pyatspi2 test suite. Callers of get_toolkit_name expect a non-NULL value (passed to strcmp), so return a default string.
atk_get_toolkit_name may return NULL when e.g. running the pyatspi2 test suite. Callers of get_toolkit_name expect a non-NULL value (passed to strcmp), so return a default string.