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/NimBLEAddress.cpp

@@ -48,6 +48,16 @@ NimBLEAddress::NimBLEAddress(ble_addr_t address) {
  * @param [in] stringAddress The hex representation of the address.
  */
 NimBLEAddress::NimBLEAddress(const std::string &stringAddress) {
+    if (stringAddress.length() == 0) {
+        memset(m_address, 0, 6);
+        return;
+    }
+
+    if (stringAddress.length() == 6) {
+        std::reverse_copy(stringAddress.data(), stringAddress.data() + 6, m_address);
+        return;
+    }
+
     if (stringAddress.length() != 17) {
         memset(m_address, 0, sizeof m_address); // "00:00:00:00:00:00" represents an invalid address
         NIMBLE_LOGD(LOG_TAG, "Invalid address '%s'", stringAddress.c_str());

src/NimBLEAddress.h

@@ -24,6 +24,7 @@
 /**************************/
 
 #include <string>
+#include <algorithm>
 
 /**
  * @brief A %BLE device address.

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;

src/NimBLEClient.h

@@ -73,7 +73,7 @@ private:
     bool                retrieveServices();  //Retrieve services from the server
 //    void                onHostReset();
 
-    NimBLEAddress    m_peerAddress = NimBLEAddress("\0\0\0\0\0\0");   // The BD address of the remote server.
+    NimBLEAddress    m_peerAddress = NimBLEAddress("");   // The BD address of the remote server.
     uint16_t         m_conn_id;
     bool             m_haveServices = false;    // Have we previously obtain the set of services from the remote server.
     bool             m_isConnected = false;     // Are we currently connected.