Check if m_pServerCallbacks is not null before calling method. (#5603)

Fixes: https://github.com/espressif/arduino-esp32/issues/5573

To reproduce:
1. Run any sample code that starts a BLE server, and does not call `setCallbacks`.
2. Connect to the device using the "LightBlue" app on iOS.
3. Observe crash shown in the issue linked above.

libraries/BLE/src/BLEServer.cpp

@@ -157,7 +157,9 @@ void BLEServer::handleGATTServerEvent(esp_gatts_cb_event_t event, esp_gatt_if_t
 
 		case ESP_GATTS_MTU_EVT:
 			updatePeerMTU(param->mtu.conn_id, param->mtu.mtu);
+			if (m_pServerCallbacks != nullptr) {
 				m_pServerCallbacks->onMtuChanged(this, param);
+			}
 			break;
 
 		// ESP_GATTS_CONNECT_EVT