[Measure] Fix seg fault in MeasurePosition::execute if subElements is empty (#22016)

* [Measure] take into account if subElements is empty

* [Measure] Remove unnecessary else block

src/Mod/Measure/App/MeasurePosition.cpp

@@ -95,7 +95,9 @@ App::DocumentObjectExecReturn* MeasurePosition::execute()
 {
     const App::DocumentObject* object = Element.getValue();
     const std::vector<std::string>& subElements = Element.getSubValues();
-
+    if (subElements.empty()) {
+        return {};
+    }
     App::SubObjectT subject {object, subElements.front().c_str()};
     auto info = getMeasureInfo(subject);