test,modem-helpers: remove unnecessary NULL check for g_free()

g_free() handles a NULL pointer properly, so there is no need to have a
NULL check before calling g_free().

src/tests/test-modem-helpers.c

@@ -3265,7 +3265,6 @@ test_cclk_response (void)
             g_assert (mm_network_timezone_get_leap_seconds (tz) == MM_NETWORK_TIMEZONE_LEAP_SECONDS_UNKNOWN);
         }
 
-        if (iso8601)
         g_free (iso8601);
 
         if (tz)