wwan: Fix signal strength display when extended signal retrieval is disabled

MMModemSignal interface is used to retrieve extended signal information that
requires periodic polling. Therefore, it needs to be manually enabled in order
to use. There if a fallback to use mm_modem_get_signal_quality when MMModemSignal
interface is unavailable, but it didn't check whether it's actually enabled,
leaving the UI with empty label.
7 jobs for wip/sadiq/wwan-panel in 2 minutes and 36 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
passed build #1412823

00:02:24

 
  Test
canceled test #1412824

00:00:11

 
  Manual
skipped asan #1412826
asan allowed to fail manual
skipped flatpak #1412825
allowed to fail manual
skipped msan #1412829
allowed to fail manual
skipped tsan #1412827
allowed to fail manual
skipped ubsan #1412828
allowed to fail manual