Do not use QStringLiteral for empty strings

The default constructor for QString is cheaper in terms of
both instructions and memory.
https://blog.qt.io/blog/2014/06/13/qt-weekly-13-qstringliteral/

src/blackgui/commandinput.cpp

@@ -24,7 +24,7 @@ namespace BlackGui
     void CCommandInput::validateCommand()
     {
         QString commandLine = text();
-        setText(QStringLiteral(""));
+        setText(QString());
 
         if (commandLine.startsWith('.'))
         {