glcolorconver: Return empty caps in transform_caps if fails.

We should not return a NULL in transform_caps() function. The NULL
will generate a assert of:
  "transform_caps returned caps which are not a real subset of the
   filter caps"
in transform base class.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/922>

ext/gl/gstglcolorconvertelement.c

@@ -149,7 +149,7 @@ gst_gl_color_convert_element_transform_caps (GstBaseTransform * bt,
   GstCaps *ret;
 
   if (base_filter->display && !gst_gl_base_filter_find_gl_context (base_filter))
-    return NULL;
+    return gst_caps_new_empty ();
 
   context = gst_gl_base_filter_get_gl_context (base_filter);