rtpbin: Return NO_PREROLL

It may contain a jitterbuffer which is no-preroll, if there is no
other element preventing the preroll (like a live source), the pipeline
will never start.
8 jobs for rtpbin-no-preroll in 9 minutes and 22 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #80833
gst indent

00:00:43

passed #80832
manifest

00:00:25

 
  Build
passed #80834
build fedora x86_64

00:04:32

 
  Test
failed #80835
check fedora

00:04:06

 
  Full Builds
skipped #80836
build android arm64 api28
skipped #80838
build cerbero cross win32
skipped #80839
build cerbero cross win64
skipped #80837
build cerbero fedora x86_64
 
Name Stage Failure
failed
check fedora Test
           Total: 2891
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-output/logs: found 3303 matching files
Uploading artifacts to coordinator... ok id=80835 responseStatus=201 Created token=AR9zFUDR
Uploading artifacts...
validate-output/logs/*.xml: found 1 matching files
Uploading artifacts to coordinator... ok id=80835 responseStatus=201 Created token=AR9zFUDR
ERROR: Job failed: exit code 1