ffmpeg: remove pointless if (nb_input_files)

It's required to be non-zero since
cc58300e

ffmpeg.c

@@ -3821,9 +3821,7 @@ static void opt_output_file(const char *filename)
         use_subtitle = file_oformat->subtitle_codec != CODEC_ID_NONE || subtitle_stream_copy || subtitle_codec_name;
         use_data = data_stream_copy ||  data_codec_name; /* XXX once generic data codec will be available add a ->data_codec reference and use it here */
 
-        /* disable if no corresponding type found and at least one
-           input file */
-        if (nb_input_files > 0) {
+        /* disable if no corresponding type found */
         check_inputs(&input_has_video,
                      &input_has_audio,
                      &input_has_subtitle,
@@ -3837,7 +3835,6 @@ static void opt_output_file(const char *filename)
             use_subtitle = 0;
         if (!input_has_data)
             use_data = 0;
-        }
 
         /* manual disable */
         if (audio_disable)    use_audio    = 0;