fix(line): Removal of partial connections around null values for step lines with connectNulls=false

[daless14]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Fixed issues

Resolves #20191

Details

Before: What was the problem?

For step line charts, connections were partially drawn between null values and other data points with "connectNulls" set to false.

After: How does it behave after the fixing?

Gaps are shown correctly for null values with no partial lines drawn.

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

N.A.

Others

Merging options

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

Other information

[echarts-bot]

Thanks for your contribution!
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[daless14]

Just wanting to check back on this PR. Is there something that should be changed/improved before it can get pulled in? Thanks!