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/Part/Gui/TaskSweep.cpp

@@ -41,6 +41,7 @@
 #include <App/Document.h>
 #include <App/DocumentObject.h>
 #include <App/Link.h>
+#include <Base/Tools.h>
 #include <Gui/Application.h>
 #include <Gui/BitmapFactory.h>
 #include <Gui/Command.h>
@@ -76,7 +77,7 @@ public:
         }
         bool allow(App::Document* /*pDoc*/, App::DocumentObject*pObj, const char*sSubName) override
         {
-            if (!sSubName || sSubName[0] == '\0') {
+            if (Base::Tools::isNullOrEmpty(sSubName)) {
                 // If selecting again the same edge the passed sub-element is empty. If the whole
                 // shape is an edge or wire we can use it completely.
                 Part::TopoShape topoShape = Part::Feature::getTopoShape(pObj);