esp32: don't try to set an environment variable to NULL

Tried to set status.net.sta.ssid to NULL.

esp32/max80/wifi.cpp

@@ -382,15 +382,16 @@ static void wifi_config_sta(void)
     WiFi.macAddress(mac);
     setenv_mac("status.net.sta.mac", mac);
 
-    setenv("status.net.sta.ssid", ssid.c_str(), 1);
     if (ssid == "") {
+	unsetenv("status.net.sta.ssid");
 	WiFi.enableSTA(false);
 	return;
     }
+    setenv("status.net.sta.ssid", ssid.c_str(), 1);
+
     sta_failure_timer = xTimerCreate("wifi_sta", configTICK_RATE_HZ*30,
 				     pdFALSE, NULL,
 				     (TimerCallbackFunction_t)sta_timeout);
-
     sta_timeout_enable();
 
     WiFi.begin(ssid.c_str(), password.c_str());