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 9 minutes and 58 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1280054
build

00:06:53

 
  Test
passed #1280055
test

00:03:04

 
  Manual
manual #1280057
asan allowed to fail manual
asan
manual #1280056
allowed to fail manual
flatpak
manual #1280060
allowed to fail manual
msan
manual #1280058
allowed to fail manual
tsan
manual #1280059
allowed to fail manual
ubsan