vaapi: plugin: Handle when no encoders/decoders available.

vaapi: plugin: Handle when no encoders/decoders available.

Nouveau driver currently only exposes postproc entry. But
vaapidecodebin is registered independent if there are decoders or not,
exposing a segmentation fault.

This patch removes the encoder/decoder/codec arrays if no entries are
found, and if no decoders are found vaapidecodebin is not
registered. Also for vaapipostproc if no postproc entry is found.

Also, if general decoder, used by vaapidecodebin, doesn't have a sink
pad string, don't register the glib type.

Fixes: #1349
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2865>