[KCModuleInfo] Don't access KPluginInfo unless it's valid

systemsettings creates a KService for .desktop files with
X-KDE-ServiceTypes=SystemSettingsExternalApp, in such cases there is no
actual module, it just launches an external app, so IIUC there is no
valid metadata and that is why constructing a KPluginInfo from that
KService fails, stepping into the code with gdb confirms this.

It makes sense to not access the KPluginInfo private member of
KCModuleInfo unless it is valid, so that service(), which is used by
some users of the class to check whether the KCModuleInfo object is
valid or not, doesn't cause a crash.

CCBUG: 421566