fix(line): avoid crash when visualMap piecewise produces no in-range stops

[JamesGoslings]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Fix a crash in line series when a visualMap piecewise piece has no upper / lower bound (e.g. { lte: null }), which previously made colorStopsInRange empty and caused Cannot read properties of undefined (reading 'coord').

Fixed issues

• [Bug] Line Charts do not work when lte is set to null #18066: [Bug] Line Charts do not work when lte is set to null

Details

Before: What was the problem?

When a visualMap of type piecewise has a piece without an upper bound (or both bounds missing), e.g.:

visualMap: {
  type: 'piecewise',
  pieces: [{ lte: null, color: 'red' }]
}

PiecewiseModel falls back the missing bound to Infinity / -Infinity, so the piece interval becomes [-Infinity, Infinity]. In PiecewiseModel.getVisualMeta, the setStop helper only fills outerColors (never stops) when an interval edge is ±Infinity. As a result the line series renderer receives an empty colorStopsInRange array and crashes at LineView.ts:350:

Uncaught TypeError: Cannot read properties of undefined (reading 'coord')
    at colorStopsInRange[0].coord - tinyExtent

Reproduction from the issue:
https://echarts.apache.org/examples/en/editor.html?c=line-simple&code=PYBwLglsB2AEC8sDeAoWsCeBBAHhAzgFywDaa6y5FsYGIApsQOQBuAhgDYCu9TANFVgBfcgF0B6HLgLEy1VNXS0GzAMZsw9AObAAThn6CR6ceQ7b60ACbEFFfAAtgAd2IAzTvnrkhE2Ft0IG2RfcjBgYA5IEFtQ9C9A-iJSQTtqZUZYJg4IaF4_amg2AFtMphBdYDcIMENqY1hTdCsNNi8wWVTBZohS6HwoftkmDHo2XX4siqqapibFfGAuXVVMuUV0NI3MMd1mACYABgBGQ7rt9HLK6triU8OCxTjtrY3R8YOT4_OLq5nb2BHB7dCjPDavRTvPZZI7HfY_bZ_G5MO4AVmBFzBigh1Chn2OAGYERskbNiATDhjtg1qKIjGI_CwCFxOABZNgxSjUEAQeirZLreQg9BaTTEKkbKKZaBcDgcR7UVSRPTMABG3F4IJpjUe4U5p0eel50A6WScgQAXjAwJwEeY3KamKsTfQJn4REIANxAA

After: How does it behave after the fixing?

In getVisualGradient of src/chart/line/LineView.ts, add a guard for the case where colorStopsInRange is empty (and stopLen is also 0, which the existing !inRangeStopLen && stopLen branch does not cover). Fall back to outerColors[0] || outerColors[1] || 'transparent' so the chart renders with a single fallback color instead of crashing.

The new branch is only hit when there are no color stops at all, so existing behaviors are unchanged.

Added a regression test piecewiseWithNullBoundOnLineSeries in test/ut/spec/component/visualMap/setOption.test.ts that reproduces the original report and asserts setOption does not throw.

Verification:

• Without the fix, the new test fails with the exact stack from the issue (LineView.ts:350).
• With the fix, the new test passes.
• All existing unit tests still pass: Test Suites: 24 passed, 24 total; Tests: 186 passed, 186 total.

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

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