avfilter/formats: set audio fmt lists for vaf filters

Currently, the logic inside the FF_FILTER_FORMATS_QUERY_FUNC branch
prevents this code from running in the event that we have a filter with
a single video input and a single audio output, as the resulting audio
output link will not have its channel counts / samplerates correctly
initialized to their default values, possibly triggering a segfault
downstream.

An example of such a filter is vaf_spectrumsynth. Although this
particular filter already sets up the channel counts and samplerates as
part of the query function and therefore avoids triggering this bug, the
bug still exists in principle. (And importantly, sets a wrong precedent)

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