group: Use proper group constructor

Otherwise we might en up having a group which is not backed by any asset
leading to possible assertion as this should never happen (see
https://gitlab.gnome.org/GNOME/pitivi/-/issues/2526)

Part-of: <gstreamer/gst-editing-services!233>
30 jobs for !233 with backport-mr-232-into-1.18-fix_group_creation in 30 minutes and 34 seconds (queued for 9 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #7815116
alpine amd64 manifest builder docker

00:00:18

passed #7815114
android docker

00:00:18

passed #7815113
cerbero fedora amd64 docker

00:00:24

passed #7815112
fedora amd64 docker

00:00:24

passed #7815117
gst-indent amd64 docker

00:00:18

 
  Preparation
passed #7815120
gst indent

00:00:08

passed #7815119
manifest

00:00:14

 
  Build
passed #7815140
gstreamer
build cerbero cross-android universal

00:11:24

passed #7815149
gst-ios-14.3
build cerbero cross-ios universal

00:19:02

passed #7815142
gstreamer
build cerbero cross win32

00:08:51

passed #7815144
gstreamer
build cerbero cross win64

00:07:40

passed #7815138
gstreamer
build cerbero fedora x86_64

00:07:49

passed #7815147
gst-macos-11.1
build cerbero macos x86_64

00:10:19

passed #7815129
build clang fedora x86_64

00:06:15

manual #7815135
docker windows 1809 allowed to fail manual
build msys2
passed #7815122
build nodebug fedora x86_64

00:05:44

passed #7815125
build static fedora x86_64

00:06:53

passed #7815128
build static nodebug fedora x86_64

00:05:32

passed #7815131
docker windows 1809
build vs2017 amd64

00:09:27

passed #7815133
docker windows 1809
build vs2017 x86

00:09:11

 
  Test
passed #7815909
gstreamer
check fedora

00:06:13

passed #7815151
gstreamer
integration testsuites fedora 1/4

00:15:21

passed #7815152
gstreamer
integration testsuites fedora 2/4

00:16:07

passed #7815153
gstreamer
integration testsuites fedora 3/4

00:15:48

passed #7815154
gstreamer
integration testsuites fedora 4/4

00:20:42

passed #7815155
gstreamer
valgrind ges

00:08:38

failed #7815150
gstreamer
check fedora

00:09:14

 
  Integrate
passed #7815156
cross-android universal examples

00:08:34

passed #7815158
gst-ios-14.3
cross-ios universal examples

00:04:31

passed #7815157
documentation

00:11:04