modemManager: improve check for CDMA system identifier

modemManager: improve check for CDMA system identifier

Upon construction of the CDMA modem proxy, _reloadCdmaOperatorName()
is called and the value of the Sid property is read.
That property is defined as UINT32 in the D-Bus interface, but the
value may not be loaded yet after the proxy is constructed, in which
case its value will be null.

In _findProviderForSid(), we'll end up calling lookup_cdma_sid(null)
which fails with the following assertion:

gnome-shell[1082]: nma_mobile_providers_database_lookup_cdma_sid: assertion 'sid > 0' failed

This commit changes the (sid == 0) check in _findProviderForSid()
to (!sid) which will also catch the null case.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/555