Replace ESM::RefId::sEmpty by default constructed RefId where possible

Static const is only required to provide a reference or a pointer when it is not
possible with default constructed temporary.

components/esm3/loadmisc.cpp

@@ -79,6 +79,6 @@ namespace ESM
         mName.clear();
         mModel.clear();
         mIcon.clear();
-        mScript = ESM::RefId::sEmpty;
+        mScript = ESM::RefId();
     }
 }