pulsesink: Deal with not being able to convert a format to caps

It is possible that PulseAudio adds formats that are not yet supported
in pulsesink, and in those cases, we want to gracefully skip them rather
than cause an assert on a NULL caps.
8 jobs for <span class="ref-name">pulse-assert-fix</span> in 15 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #100163
gst indent

00:00:32

passed #100162
manifest

00:00:39

 
  Build
passed #100164
build fedora x86_64

00:02:28

 
  Test
passed #100165
check fedora

00:03:11

 
  Full Builds
passed #100166
build android arm64 api28

00:03:44

passed #100168
build cerbero cross win32

00:08:43

passed #100169
build cerbero cross win64

00:06:00

passed #100167
build cerbero fedora x86_64

00:07:05