cinterion: if user OR password given, don't set the other as (null)

[modem2/ttyACM1/at] --> 'AT^SGAUTH=8,1,t-d1,(null)<CR>'
  [modem2/ttyACM1/at] <-- '<CR><LF>+CME ERROR: 4<CR><LF>'

We should use an empty string instead.

plugins/cinterion/mm-broadband-bearer-cinterion.c

@@ -236,7 +236,11 @@ build_auth_string (MMBroadbandBearerCinterion *self,
         encoded_auth = BEARER_CINTERION_AUTH_PAP;
     }
 
-    return g_strdup_printf ("^SGAUTH=%u,%d,%s,%s", cid, encoded_auth, passwd, user);
+    return g_strdup_printf ("^SGAUTH=%u,%d,%s,%s",
+                            cid,
+                            encoded_auth,
+                            passwd ? passwd : "",
+                            user ? user : "");
 }
 
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