lavfi: rewrite nullsrc using the code in vsrc_testsrc.c

lavfi: rewrite nullsrc using the code in vsrc_testsrc.c

Factorize code, extend the functionality of the filter, and make it
return empty buffers. This is useful for filters which ignore the input
frames content.

This is also changing the syntax of the nullsrc source, and dropping the
framerate expression evaluation, which does not look particularly useful.