Fix bug preventing filtering named constraints (#19339)

* Fix bug preventing filtering named constraints

There was a bug (see issue #11843) that displayed a filter for Named
constraints in the Sketcher Workbench whose toggling did not affect the
UI in any way. This enables this feature by checking if a given
constraint has a custom name or not and then appropriately toggling its
visibility.

* Use `std::string::empty()` per reviewer suggestion

Co-authored-by: Benjamin Nauck <benjamin@nauck.se>

---------

Co-authored-by: Chris Hennes <chennes@pioneerlibrarysystem.org>
Co-authored-by: Benjamin Nauck <benjamin@nauck.se>

src/Mod/Sketcher/Gui/TaskSketcherConstraints.cpp

@@ -1649,6 +1649,8 @@ bool TaskSketcherConstraints::isConstraintFiltered(QListWidgetItem* item)
                 break;
         }
 
+        visible |= !constraint->Name.empty() && checkFilterBitset(multiFilterStatus, FilterValue::Named);
+
         // Then we re-filter based on selected/associated if such mode selected.
         if (visible && specialFilterMode == SpecialFilterType::Selected) {
             visible = (std::find(selectionFilter.begin(), selectionFilter.end(), it->ConstraintNbr)