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/plugins/nim/project/nimtoolchain.cpp

@@ -71,7 +71,7 @@ Abi NimToolChain::targetAbi() const
 
 bool NimToolChain::isValid() const
 {
-    if (m_compilerCommand.isNull())
+    if (m_compilerCommand.isEmpty())
         return false;
     QFileInfo fi = compilerCommand().toFileInfo();
     return fi.isExecutable();