fix(axis): Reverse default splitArea color order

[mynk2611]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Reverses the default color order of axis.splitArea to fix a rendering bug on heatmaps with empty data.This fix works for both the Canvas and SVG renderers.

Fixed issues

• [Bug] 热力图对于空数据的表现不一致 #21290

Details

Before: What was the problem?

The color of the top-left block is inconsistent with the rest of its column.

After: How does it behave after the fixing?

The color is now consistent for both SVG and Canvas renderers.

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

Security Checking

• This PR uses security-sensitive Web APIs.

ZRender Changes

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

Related test cases or examples to use the new APIs

N.A.

Merging options

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

Other information

[echarts-bot]

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