return macaddress (#2477)

* WiFi.macaddress() returns mac address

* change description

* return macaddress when WiFi mode is WIFI_MODE_NULL

libraries/WiFi/src/WiFiSTA.cpp

 /*
- ESP8266WiFiSTA.cpp - WiFi library for esp8266
+ WiFiSTA.cpp - WiFi library for esp32
 
  Copyright (c) 2014 Ivan Grokhotkov. All rights reserved.
  This file is part of the esp8266 core for Arduino environment.
@@ -418,6 +418,9 @@ uint8_t* WiFiSTAClass::macAddress(uint8_t* mac)
     if(WiFiGenericClass::getMode() != WIFI_MODE_NULL){
         esp_wifi_get_mac(WIFI_IF_STA, mac);	
     }
+    else{
+        esp_read_mac(mac, ESP_MAC_WIFI_STA);
+    }
     return mac;
 }
 
@@ -430,10 +433,11 @@ String WiFiSTAClass::macAddress(void)
     uint8_t mac[6];
     char macStr[18] = { 0 };
     if(WiFiGenericClass::getMode() == WIFI_MODE_NULL){
-        return String();
+        esp_read_mac(mac, ESP_MAC_WIFI_STA);
     }
+    else{
         esp_wifi_get_mac(WIFI_IF_STA, mac);
-
+    }
     sprintf(macStr, "%02X:%02X:%02X:%02X:%02X:%02X", mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
     return String(macStr);
 }