fix: fix new node invisible after invoke graph.clear

[Aarebecca]

• 修复调用 Graph.clear 后重新添加节点时，新节点不可见的问题

---

• Fix the problem that new nodes are not visible when nodes are added again after calling Graph.clear.

Issues:

• [Bug]: After clearing the canvas and adding a node again, the node will display an exception #6649

[petercat-assistant]

Walkthrough

This pull request addresses an issue where new nodes become invisible after invoking Graph.clear and then adding nodes again. The changes ensure that the graph is properly cleared and redrawn, making new nodes visible.

Changes

File	Summary
packages/g6/__tests__/bugs/graph-draw-after-clear.spec.ts	Added a test to verify that the graph redraws correctly after being cleared and new data is added.
packages/g6/__tests__/snapshots/bugs/graph-draw-after-clear/default.svg	Added a snapshot for the test case to ensure visual correctness.
packages/g6/src/runtime/data.ts	Removed warning comments from the getChanges method.
packages/g6/src/runtime/element.ts	Added a clear method to remove all elements and reset styles, and updated the destroy method to use clear.
packages/g6/src/runtime/graph.ts	Updated the clear method to reset the model and element states, ensuring proper clearing of the graph.

[petercat-assistant]

Ensure that the element?.clear() method is correctly handling all elements and resetting the necessary states. This is crucial for the functionality of clearing and redrawing the graph.