gdbus-wrapper: Guard against emitting signals when object isn't exported

g_dbus_interface_skeleton_get_connection() can return null if the
object isn't exported on the bus. In those cases, refrain from trying to
emit signals.

Closes: #332
16 jobs for 332-fix-dbus-connection in 3 minutes and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Source Check
passed #849669
build_recommended

00:01:03

passed #849670
cppcheck

00:00:43

passed #849671
cpplint

00:01:19

passed #849672
eslint

00:00:49

passed #849673
iwyu

00:00:30

 
  Test
passed #849675
build_maximal

00:01:35

passed #849676
build_minimal

00:00:59

passed #849674
asan
sanitizer_gcc

00:02:04

 
  Manual
manual #849677
allowed to fail manual
coverage
manual #849680
allowed to fail manual
installed_tests
manual #849678
allowed to fail manual
iwyu-full
manual #849679
asan allowed to fail manual
sanitizer_clang
manual #849681
allowed to fail manual
valgrind
manual #849684
allowed to fail manual
zeal_11
manual #849682
allowed to fail manual
zeal_2
manual #849683
allowed to fail manual
zeal_4