voice: run downlink processing hook when gaps in downlink
voice: run downlink processing hook when gaps in downlink

Run the voice downlink processing hook even if the data chunk is
silent (as per pa_memblock_is_silence()). This is important to
correctly handle both NODATA/DTX frames coming from the modem, as
well as cases where no frames are coming at all from the modem
(handovers, call hold, and so forth).

Partial fix to NB#236602.

nemomobile/pulseaudio-module-cmtspeech-n9xx,jusa/pulseaudio-modules-nemo,jusa/pulseaudio-modules-nemo,maemo-foss/maemo-multimedia-pulseaudio-modules-meego,maemo-foss/maemo-multimedia-pulseaudio-modules-meego,nemomobile/pulseaudio-module-cmtspeech-n9xx,jusa/pulseaudio-modules-nemo,maemo-foss/maemo-multimedia-pulseaudio-modules-meego