audioconvert: check if mix-matrix is set rather than checking its dimensions.

Since !481 mix-matrix can have proper dimensions but still be considered not set.
35 jobs for !957 with audioconvert-check-mix-matrix-is-set in 49 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build Docker
passed #5950761
alpine amd64 manifest builder docker

00:00:17

passed #5950760
android docker

00:00:35

passed #5950759
cerbero fedora amd64 docker

00:00:18

passed #5950758
fedora amd64 docker

00:00:18

passed #5950762
gst-indent amd64 docker

00:00:31

 
  Preparation
passed #5950765
check allow-edit

00:00:09

passed #5950764
gst indent

00:00:13

manual #5950763
manual
manifest
 
  Build
created #5950774
gstreamer
build cerbero cross-android universal
created #5950778
gst-ios-13.2
build cerbero cross-ios universal
created #5950775
gstreamer
build cerbero cross win32
created #5950776
gstreamer
build cerbero cross win64
created #5950773
gstreamer
build cerbero fedora x86_64
created #5950777
gst-macos-10.15
build cerbero macos x86_64
created #5950779
1809 gstreamer-windows windows docker
build cerbero msvc x86_64
created #5950769
build clang fedora x86_64
created #5950772
1809 windows docker allowed to fail manual
build msys2
created #5950766
build nodebug fedora x86_64
created #5950767
build static fedora x86_64
created #5950768
build static nodebug fedora x86_64
created #5950770
1809 windows docker
build vs2017 amd64
created #5950771
1809 windows docker
build vs2017 x86
 
  Test
created #5950780
gstreamer
check fedora
created #5950781
gstreamer
integration testsuites fedora 1/4
created #5950782
gstreamer
integration testsuites fedora 2/4
created #5950783
gstreamer
integration testsuites fedora 3/4
created #5950784
gstreamer
integration testsuites fedora 4/4
created #5950788
gstreamer
valgrind bad
created #5950785
gstreamer
valgrind base
created #5950789
gstreamer
valgrind ges
created #5950786
gstreamer
valgrind good
created #5950787
gstreamer
valgrind ugly
 
  Integrate
created #5950790
cross-android universal examples
created #5950792
gst-ios-13.2
cross-ios universal examples
created #5950791
documentation