get_spn_done(): Handle the case of an entirely empty SPN correctly.

BUG=chrome-os-partner:4595
TEST=Insert a SIM with a present but empty (all 0xFF) SPN and check
the system log for a (lack of) assertion errors from mm_charset_gsm_unpacked_to_utf8().

Change-Id: I3fcddd6d44cbd11c7e84d41da1ffa919f6916669
Reviewed-on: http://gerrit.chromium.org/gerrit/3139
Reviewed-by: Eric Shienbrood <ers@chromium.org>
Tested-by: Nathan J. Williams <njw@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/3248
Reviewed-by: Nathan J. Williams <njw@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/3266
1 file changed