mxfvanc: Handle empty ANC essence

Not having any *actual* ANC is totally fine and common usage with several MXF
variants.

In order to properly advance the streams, the essence handler returns an empty
GAP buffer which gets converted to a GST_EVENT_GAP.

Part-of: <gstreamer/gst-plugins-bad!2387>
29 jobs for !2387 with backport-mr-2345-into-1.18-mxf-empty-vanc in 24 minutes and 32 seconds (queued for 4 seconds)
detached
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #11625875

00:00:29

passed android docker #11625874

00:00:30

passed cerbero fedora amd64 docker #11625873

00:00:43

passed fedora amd64 docker #11625872

00:00:44

passed gst-indent amd64 docker #11625876

00:00:56

 
  Preparation
passed gst indent #11625878

00:00:21

passed manifest #11625877

00:00:36

 
  Build
passed build cerbero cross-android universal #11625887
gstreamer

00:09:04

passed build cerbero cross-ios universal #11625891
gst-ios-14.3

00:19:01

passed build cerbero cross win32 #11625888
gstreamer

00:05:11

passed build cerbero cross win64 #11625889
gstreamer

00:05:16

passed build cerbero fedora x86_64 #11625886
gstreamer

00:05:15

passed build cerbero macos x86_64 #11625890
gst-macos-11.1

00:09:44

passed build clang fedora x86_64 #11625882

00:05:05

manual build msys2 #11625885
docker windows 1809 allowed to fail manual
passed build nodebug fedora x86_64 #11625879

00:06:10

passed build static fedora x86_64 #11625880

00:06:10

passed build static nodebug fedora x86_64 #11625881

00:06:10

passed build vs2017 amd64 #11625883
docker windows 1809

00:09:24

passed build vs2017 x86 #11625884
docker windows 1809

00:09:08

 
  Test
passed check fedora #11625892
gstreamer

00:06:30

passed integration testsuites fedora 1/4 #11625893
gstreamer

00:07:17

passed integration testsuites fedora 2/4 #11625894
gstreamer

00:08:02

passed integration testsuites fedora 3/4 #11625895
gstreamer

00:07:45

passed integration testsuites fedora 4/4 #11625896
gstreamer

00:05:50

passed valgrind bad #11625897
gstreamer

00:06:28

 
  Integrate
passed cross-android universal examples #11625898

00:06:36

passed cross-ios universal examples #11625900
gst-ios-14.3

00:03:53

passed documentation #11625899

00:12:07