Correct error in creation of NimBLEAddress from six bytes in a std::string

* Add creation of a NimBLEAddress from a std::string of 6 exactly bytes.

* Add creation of a NimBLEAddress from an empty string.

src/NimBLEAdvertisedDevice.h

@@ -95,7 +95,7 @@ private:
     bool m_haveTXPower;
 
 
-    NimBLEAddress  m_address = NimBLEAddress("\0\0\0\0\0\0");
+    NimBLEAddress  m_address = NimBLEAddress("");
     uint8_t         m_advType;
     uint16_t        m_appearance;
     int             m_deviceType;