[ModemManager] PATCH: Fix a corner case of SPN handling

[Nathan Williams <njw google com>]

This fixes a glitch where we return NULL (and log an assertion
failure) rather than returning "" for a fully-empty SPN string, as
seen on some European SIM cards (most recently tested with a Vodafone
UK SIM).

    - Nathan

Attachment: 0002-get_spn_done-Handle-the-case-of-an-entirely-empty-SP.patch
Description: Binary data