LineChartOpDesc.generatePythonCode throws NPE when lines is null (the default)

[Yicong-Huang]

What happened?

common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/visualization/lineChart/LineChartOpDesc.scala declares var lines: util.List[LineConfig] = _, which defaults to null. createPlotlyFigure then calls lines.asScala.map(...) without a null check, so generatePythonCode on a freshly constructed LineChartOpDesc raises java.lang.NullPointerException at LineChartOpDesc.scala:68 — quite different from the explicit AssertionError paths used by sibling visualizers (e.g. HeatMapOpDesc, BarChartOpDesc).

The fix is either to default lines to an empty list, or to null-guard / assert nonEmpty before the .asScala.map(...) call.

How to reproduce?

import org.apache.texera.amber.operator.visualization.lineChart.LineChartOpDesc

new LineChartOpDesc().generatePythonCode()
// java.lang.NullPointerException
//   at LineChartOpDesc.createPlotlyFigure(LineChartOpDesc.scala:68)
//   at LineChartOpDesc.generatePythonCode(LineChartOpDesc.scala:122)

Version

1.1.0-incubating (Pre-release/Master)

[Ma77Ball]

/take