tagdemux: no input data implies no type can be found

... and posting a proper error message to this effect is appropriately
informative and prevents auto-plugging otherwise stalling.

gst-libs/gst/tag/gsttagdemux.c

@@ -1196,7 +1196,9 @@ gst_tag_demux_sink_activate (GstPad * sinkpad)
       demux->priv->strip_start + demux->priv->strip_end) {
     /* There was no data (probably due to a truncated file) */
     GST_DEBUG_OBJECT (demux, "No data in file");
-    return FALSE;
+    /* so we don't know about type either */
+    GST_ELEMENT_ERROR (demux, STREAM, TYPE_NOT_FOUND, (NULL), (NULL));
+    goto done_activate;
   }
 
   /* 3 - Do typefinding on data */