Fix #117191: Crash using the Extrude tool and Snap Base

`GIZMO_GT_button_2d` has no modal/custom_modal callback function
defined hence the crash due to calling `modal_fn()`

Pull Request: https://projects.blender.org/blender/blender/pulls/117194

source/blender/editors/transform/transform_mode_snapsource.cc

@@ -237,7 +237,9 @@ void transform_mode_snap_source_init(TransInfo *t, wmOperator * /*op*/)
     const wmEvent *event = CTX_wm_window(t->context)->eventstate;
 #  ifdef RESET_TRANSFORMATION
     wmGizmoFnModal modal_fn = gz->custom_modal ? gz->custom_modal : gz->type->modal;
-    modal_fn(t->context, gz, event, eWM_GizmoFlagTweak(0));
+    if (modal_fn) {
+      modal_fn(t->context, gz, event, eWM_GizmoFlagTweak(0));
+    }
 #  endif
 
     WM_gizmo_modal_set_while_modal(t->region->gizmo_map, t->context, nullptr, event);