broadband-modem-mbim: fix crash with missing ICCID

Output fields like ICCID or IMSI are optional in MBIM Subscriber Ready Status v3 operations. We should not assume they are set to a valid value when the operation succeeds.

2023-10-27T10:24:16.512461Z [modem0] started checking for basic SIM details... 2023-10-27T10:24:16.516104Z mm_3gpp_parse_iccid: assertion 'raw_iccid != NULL' failed