fix(tooltip): fix potential NPE when setting option with notMerge strategy

[ktx-abhay]

• Fix [Bug] Cannot read properties of null (reading 'offsetWidth') when using notMerge #20335
• Optional chaining for tooltip offsetWidth and offsetHeight

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
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What does this PR do?

Fixed issues

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ZRender Changes

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

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[echarts-bot]

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[github-actions]

The changes brought by this PR can be previewed at: https://echarts.apache.org/examples/editor?version=PR-20435@50b715b

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