Modernize source code: last part (#6285)

* Modernize source code: last par

* Use empty when needed
* Use emplace_back instead of push_back when needed
* For range-based loops
* Initializers fixes
* constructors, destructors default
* c++ C stl includes

src/network/connection.cpp

@@ -1281,7 +1281,7 @@ bool ConnectionSendThread::packetsQueued()
 			continue;
 
 		for (Channel &channel : (dynamic_cast<UDPPeer *>(&peer))->channels) {
-			if (channel.queued_commands.size() > 0) {
+			if (!channel.queued_commands.empty()) {
 				return true;
 			}
 		}