Fix Krazy warnings about QString::null/clear().

Change-Id: I11faa5af0c186a9126c4af6f86e56250030d6cd9
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>

src/libs/qmljs/qmljscompletioncontextfinder.cpp

@@ -308,7 +308,7 @@ void CompletionContextFinder::checkImport()
         if (m_libVersion.isNull())
             m_libVersion = QLatin1String("");
     } else {
-        m_libVersion = QString();
+        m_libVersion.clear();
     }
 }