audiodecoder / videodecoder: Don't unconditionally post an error message if no frames were decoded before end-of-stream
This can also happen correctly in dynamic pipeline when switching between streams very fast, and switching just after the caps event and before the first decoded buffer can be output.
It should probably get at least a property instead of always posting an error message, but maybe someone has an idea how to do some meaningful autodetection here if there was an error or not?