If there is no movement, DragStart is not triggered. (#17233)

# Objective

Fixed the issue where DragStart was triggered even when there was no
movement
https://github.com/bevyengine/bevy/issues/17230

## Solution

- When position delta is zero, don't trigger DragStart events, DragStart
is not triggered, so DragEnd is not triggered either. Everything is
fine.

## Testing

- tested with the code from the issue

---

## Migration Guide

> Fix the missing part of Drag
https://github.com/bevyengine/bevy/pull/16950

crates/bevy_picking/src/events.rs

@@ -662,6 +662,9 @@ pub fn pointer_events(
             }
             // Moved
             PointerAction::Moved { delta } => {
+                if delta == Vec2::ZERO {
+                    continue; // If delta is zero, the following events will not be triggered.
+                }
                 // Triggers during movement even if not over an entity
                 for button in PointerButton::iter() {
                     let state = pointer_state.get_mut(pointer_id, button);