basetransform: fix critical if transform_caps() returned NULL
klass->transform_caps() may return NULL, which was raising this critical:
GStreamer-CRITICAL **: 12:23:56.243: gst_caps_is_subset: assertion 'subset != NULL' failed
klass->transform_caps() may return NULL, which was raising this critical:
GStreamer-CRITICAL **: 12:23:56.243: gst_caps_is_subset: assertion 'subset != NULL' failed