fix(lineChart): prevent NullPointerException when lines is not configured

[Ma77Ball]

What changes were proposed in this PR?

LineChartOpDesc declared var lines: util.List[LineConfig] = _, which defaults to null in Scala/Java. Calling generatePythonCode() on a freshly constructed instance immediately crashed with a NullPointerException at
createPlotlyFigure because .asScala.map(...) was called on a null reference.

Two changes:

• Default lines to new util.ArrayListLineConfig instead of null, so it is always safe to iterate.
• Add assert(lines.asScala.nonEmpty, "At least one line must be configured") at the top of createPlotlyFigure(), matching the explicit assertion pattern used by sibling visualizers (HeatMapOpDesc, BarChartOpDesc).

Any related issues, documentation, or discussions?

#4811

How was this PR tested?

Unit tests in LineChartOpDescSpec cover:

• Default-constructed LineChartOpDesc throws AssertionError with a descriptive message when lines is empty
• Explicitly setting lines to an empty list also throws AssertionError
• A fully configured operator with at least one LineConfig generates valid Python code containing the expected Plotly structure

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude opus 4.6 in compliance with ASF

[Copilot]

Pull request overview

This PR updates the Line Chart visualization operator to fail fast when no line series are configured, instead of crashing with a NullPointerException during Python code generation. It fits into the visualization operator code by aligning LineChartOpDesc's configuration handling with the validation style used by similar chart operators.

Changes:

• Initialize LineChartOpDesc.lines with an empty ArrayList instead of leaving it null.
• Add a non-empty assertion in createPlotlyFigure() before generating Plotly traces.
• Update LineChartOpDescSpec to expect AssertionError for default and explicitly empty lines configurations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/visualization/lineChart/LineChartOpDesc.scala	Changes Line Chart config initialization and adds fail-fast validation before trace generation.
common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/lineChart/LineChartOpDescSpec.scala	Updates unit tests to cover the new empty/default lines validation behavior.

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[Ma77Ball]

@aglinxinyuan please review.