[App] add a missing return statement (#6519)

* [App] add a missing return statement

as discussed here: https://github.com/FreeCAD/FreeCAD/commit/c96f61b0a1e3ab15cc2cb198fdbe58ce4a5d8c59#commitcomment-68066635

src/App/AutoTransaction.h

@@ -31,7 +31,9 @@ class Application;
 class AppExport AutoTransaction {
 private:
     /// Private new operator to prevent heap allocation
-    void* operator new(size_t size);
+    void* operator new(size_t size) {
+        return nullptr;
+    };
 
 public:
     /** Constructor
@@ -120,7 +122,9 @@ public:
 
 private:
     /// Private new operator to prevent heap allocation
-    void* operator new(size_t size);
+    void* operator new(size_t size) {
+        return nullptr;
+    };
 
 private:
     bool active;