Part: fix Sweep path selection

Fixes #0004255

src/Mod/Part/Gui/TaskSweep.cpp

@@ -84,7 +84,7 @@ public:
         bool allow(App::Document* /*pDoc*/, App::DocumentObject*pObj, const char*sSubName)
         {
             if (pObj->getTypeId().isDerivedFrom(Part::Feature::getClassTypeId())) {
-                if (!sSubName) {
+                if (!sSubName || sSubName[0] == '\0') {
                     // 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.
                     const TopoDS_Shape& shape = static_cast<Part::Feature*>(pObj)->Shape.getValue();