Base: Add isNullOrEmpty string helper

This adds isNullOrEmpty string helper that cheks if string is... well
null or empty. It is done to improve readability of the code and better
express intent.

src/Mod/PartDesign/Gui/ReferenceSelection.cpp

@@ -35,6 +35,7 @@
 #include <App/Origin.h>
 #include <App/Datums.h>
 #include <App/Part.h>
+#include <Base/Tools.h>
 #include <Gui/Command.h>
 #include <Gui/Document.h>
 #include <Gui/MainWindow.h>
@@ -86,7 +87,7 @@ bool ReferenceSelection::allow(App::Document* pDoc, App::DocumentObject* pObj, c
     }
 #endif
     // Handle selection of geometry elements
-    if (!sSubName || sSubName[0] == '\0')
+    if (Base::Tools::isNullOrEmpty(sSubName))
         return type.testFlag(AllowSelection::WHOLE);
 
     // resolve links if needed