feat(line): show single symbol when pre and next data is null

[linghaoSu]

Brief Information

This pull request is in the type of:

• [ ] bug fixing
• [x] new feature
• [ ] others

What does this PR do?

when showSymbol is 'single', add symbol when pre data and next data is null

Fixed issues

• fix #17708

Details

Before: What was the problem?

when showSymbol is false, when pre and next is null, the data between would not show

normal

hover:

After: How does it behave after the fixing?

will show single symbol when pre and next data is null, unless connectNulls is true

Document Info

One of the following should be checked.

• [ ] This PR doesn't relate to document changes
• [x] 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

N.A.

Others

Merging options

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

Other information