WM: avoid unnecessary undo step creation when duplicating

Calling duplicate operation without selecting anything registers an undo
step. If nothing is selected (keyframe, curve, object, etc.), cancel the
operator execution to prevent undo push.

Patch improves following operators:

- ACTION_OT_duplicate
- GPENCIL_OT_duplicate
- GRAPH_OT_duplicate
- MESH_OT_duplicate
- NODE_OT_duplicate
- OBJECT_OT_duplicate

Reviewed By: campbellbarton

Ref D14511

source/blender/editors/space_graph/graph_edit.c

@@ -623,11 +623,12 @@ void GRAPH_OT_paste(wmOperatorType *ot)
 /** \name Duplicate Keyframes Operator
  * \{ */
 
-static void duplicate_graph_keys(bAnimContext *ac)
+static bool duplicate_graph_keys(bAnimContext *ac)
 {
   ListBase anim_data = {NULL, NULL};
   bAnimListElem *ale;
   int filter;
+  bool changed = false;
 
   /* Filter data. */
   filter = (ANIMFILTER_DATA_VISIBLE | ANIMFILTER_CURVE_VISIBLE | ANIMFILTER_FOREDIT |
@@ -636,13 +637,15 @@ static void duplicate_graph_keys(bAnimContext *ac)
 
   /* Loop through filtered data and delete selected keys. */
   for (ale = anim_data.first; ale; ale = ale->next) {
-    duplicate_fcurve_keys((FCurve *)ale->key_data);
+    changed |= duplicate_fcurve_keys((FCurve *)ale->key_data);
 
     ale->update |= ANIM_UPDATE_DEFAULT;
   }
 
   ANIM_animdata_update(ac, &anim_data);
   ANIM_animdata_freelist(&anim_data);
+
+  return changed;
 }
 
 /* ------------------- */
@@ -657,7 +660,9 @@ static int graphkeys_duplicate_exec(bContext *C, wmOperator *UNUSED(op))
   }
 
   /* Duplicate keyframes. */
-  duplicate_graph_keys(&ac);
+  if (!duplicate_graph_keys(&ac)) {
+    return OPERATOR_CANCELLED;
+  }
 
   /* Set notifier that keyframes have changed. */
   WM_event_add_notifier(C, NC_ANIMATION | ND_KEYFRAME | NA_ADDED, NULL);