validate: fix crash if timeout when media_descriptor is None

Some tests may not have any media_descriptor. If those were failing to
shutdown after EOS we were calling get_protocol() on None.
6 jobs for media-none in 31 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #65051
gst indent

00:00:14

passed #65050
manifest

00:00:19

 
  Build
passed #65052
build fedora x86_64

00:02:52

 
  Test
passed #65053
check fedora

00:15:58

 
  Full Builds
passed #65054
build android arm64 api28

00:02:16

passed #65055
build cerbero fedora x86_64

00:10:07