panel-loader: Skip static init func if the panel has not been added

For several reasons a panel cannot be added to the model (e.g. if
OnlyShowIn/NotShowIn is set to skip the panel on a particular desktop). If this
is the case we should avoid calling the panel's static init func (if any).

Fixes: GNOME/gnome-control-center#248
11 jobs for fix-lp-1799675 in 4 minutes and 25 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #131722
build

00:01:58

 
  Test
passed #131724
asan

00:01:43

passed #131723
test

00:00:43

 
  Manual
manual #131729
allowed to fail manual
armv8
manual #131725
allowed to fail manual
flatpak
manual #131728
allowed to fail manual
msan
manual #131730
allowed to fail manual
ppc64le
manual #131726
allowed to fail manual
tsan
manual #131727
allowed to fail manual
ubsan
 
  Review
manual #131731
allowed to fail manual
review
manual #131732
allowed to fail manual
stop_review