Utils: Remove FileName::isNull

... and its uses in order to shrink the FileName interface.

Change-Id: I4b9a229d98ad53c4f189e381c89241981ddbed2e
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/fileutils.h

@@ -97,7 +97,6 @@ public:
 
     using QString::clear;
     using QString::isEmpty;
-    using QString::isNull;
 
 private:
     FileName(const QString &string);