Revert "Merge commit '8b830ee9'" (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 3e3779cd, reversing
changes made to 0b28039a.
Reviewed-by: Ganesh Ajjanagadde <gajjanag@mit.edu>