fix(custom): fix custom elements probably can't be removed due to NPE when applying leave transition

[plainheart]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Fix potential null pointer exception when applying leave transition on removed elements.

Fixed issues

• Resolves [Bug] Regression between 5.2.2 and 5.3.0 on custom series #17185

Details

Before: What was the problem?

The elements probably can't be removed due to the unexpected NPE when applying leave transition.

After: How does it behave after the fixing?

Detect if the targeted element exists before applying leave transition.

Document Info

One of the following should be checked.

• This PR doesn't relate to document changes
• The document should be updated later
• The document changes have been made in apache/echarts-doc#xxx

Misc

ZRender Changes

• This PR depends on ZRender changes (ecomfe/zrender#xxx).

Related test cases or examples to use the new APIs

Please refer to the first case in test/custom.html

Others

Merging options

• Please squash the commits into a single one when merging.

Other information

This was brought in the commit 393eb30. (#16225)

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[echarts-bot]

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