feat(matrix): support matrix.x/y.length for conveniently creating a headless matrix without composing an array

[100pah]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

A tiny update:

Support matrix.x/y.length for conveniently create matrix without composing an array when header is not needed.

Before:

matrix: {
    x: {data: Array(5).fill(null)},
    y: {data: Array(3).fill(null)},
}

After:

// Support this way, more straightforward.
matrix: {
    x: {length: 5},
    y: {length: 3},
}

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

TODO Doc: Need to clarify that:
If neither matrix.x.data nor matrix.x.length exists, it collect values from series.data or dataset.source.

[github-actions]

The changes brought by this PR can be previewed at: https://echarts.apache.org/examples/editor?version=PR-21191@5e54f9d

[echarts-bot]

Document changes are required in this PR. Please also make a PR to apache/echarts-doc for document changes and update the issue id in the PR description. When the doc PR is merged, the maintainers will remove the PR: awaiting doc label.

[echarts-bot]

Congratulations! Your PR has been merged. Thanks for your contribution! 👍