sip: remove annoying GCC warnings

Refs #52750

Change-Id: Ie2ad357c668ff80e7c489f54f0c9545efeee3981

daemon/src/sip/sipaccount.h

@@ -582,7 +582,7 @@ class SIPAccount : public SIPAccountBase {
         /**
          * The STUN server name (hostname)
          */
-        pj_str_t stunServerName_ {};
+        pj_str_t stunServerName_ {nullptr, 0};
 
         /**
          * The STUN server port

daemon/src/sip/sipaccountbase.h

@@ -147,7 +147,7 @@ public:
         return false;
     }
 
-    virtual pj_str_t getStunServerName() const { return pj_str_t {}; };
+    virtual pj_str_t getStunServerName() const { return pj_str_t {nullptr, 0}; };
 
     virtual pj_uint16_t getStunPort() const { return 0; };