iface-modem-3gpp: avoid to connect signal on NULL SIM object
Happens when we boot without a SIM card inserted:
ModemManager[12065]: [1679912011.887410] [ttyACM2/at] --> 'AT+CLCK="PC",2' ModemManager[12065]: [1679912011.905401] [ttyACM2/at] <-- '+CLCK: 0OK' (ModemManager:12065): GLib-GObject-CRITICAL **: 12:13:31.905: invalid (NULL) pointer instance