Some used 'is_valid()' checks, others not. Validity is already checked in 'unref()',
and 'remove_resource_format_*()' has an ERR_FAIL condition on 'is_null()' already
(which shouldn't happen since we're only unregistering things that we previously
registered.

Also add missing GDCLASS statement in ResourceFormatLoaderVideoStreamGDNative,
missed in #20552 which was last amended before #19501 was merged.