Server: Don't warn on empty --serverinfo

The intention of 5b2c2de was to introduce logging for bad --serverinfo
parameters. However, it also warns about bad serverinfo if no
--serverinfo is given at all. This is confusing.
Therefore, this commit removes the error when no --serverinfo has been
given.

Fixes: #2947

src/serverlist.cpp

@@ -234,7 +234,10 @@ CServerListManager::CServerListManager ( const quint16  iNPortNum,
     }
     else
     {
-        qWarning() << "Ignoring invalid serverinfo, please verify the parameter syntax.";
+        if ( !strServerInfo.isEmpty() )
+        {
+            qWarning() << "Ignoring invalid serverinfo, please verify the parameter syntax.";
+        }
     }
 
     // per definition, the very first entry is this server and this entry will