🐛 [BUG]`customContent` will pass `null` argument on the first fire.

### 🐛 Bug description [Please make everyone to understand it]

> Please provide a link by forking these links [G2Plot](https://codesandbox.io/s/g2plot-issue-template-gzfdq?file=/index.ts) or GitHub repo, a minimal reproduction.

* _Required_ **Link to minimal reproduction**: https://codesandbox.io/s/customcontent-issue-xi2jdo?file=/index.ts

### 📷 Step to reproduce

1. Open the codesandbox link.
2. Open Chrome Devtools.
3. Check the argument when pasued.

### 🏞 Expected result

Do not fire `customContent` when the argument is `null`, or document it if it's hard to solve at now.

### 🚑 Any additional [like screenshots]

* **G2Plot Version**: `2.3.39`
* **Platform**: MacOS

Start with the linked CodeSandbox reproduction and inspect the G2Plot source for the customContent callback path. Confirm when the first fire receives null, then ensure the behavior matches the expected result and verify it against the reproduction.

Written by the indexing model from the issue text.