gio-tool-info: Prevent criticals if mount options are not available
NULL is valid return value for the
because mount options are currently provided only by libmount implementation.
gio tool passes the returned value to the
without checking, which produces the following critical warning:
GLib-CRITICAL **: 13:47:15.294: g_strescape: assertion 'source != NULL' failed
Let's add the missing check to prevent the critical warnings.