Revert "Merge commit '8b830ee9a26d47b138f12a82085cdb372f407f1e'" (avconv: Do not try to configure filter outputs without streams)

FFmpeg already tests for this case in configure_output_filter() and printed a
clearer error message

example:
./ffmpeg -f lavfi -i color -f lavfi -i color -filter_complex "[1]null[x],[0][1]overlay" -f null -
before the merge / after the revert:
Filter null has a unconnected output

after the merge / before the revert:
Output pad "default" with type video of the filter instance "Parsed_null_0" of null not connected to any destination
Error configuring complex filters.
Invalid argument

This reverts commit 3e3779cd517e4d2d1f21d5eb6afdb428cd3c1aa0, reversing
changes made to 0b28039a44.

Reviewed-by: Ganesh Ajjanagadde <gajjanag@mit.edu>

ffmpeg_filter.c

@@ -1035,12 +1035,8 @@ int configure_filtergraph(FilterGraph *fg)
         }
     avfilter_inout_free(&inputs);
 
-    for (cur = outputs, i = 0; cur; cur = cur->next, i++) {
-        OutputFilter *ofilter = fg->outputs[i];
-        if (ofilter->ost)
-            configure_output_filter(fg, ofilter, cur);
-    }
-
+    for (cur = outputs, i = 0; cur; cur = cur->next, i++)
+        configure_output_filter(fg, fg->outputs[i], cur);
     avfilter_inout_free(&outputs);
 
     if ((ret = avfilter_graph_config(fg->graph, NULL)) < 0)