When the user toggles "Show Curve" off, updateCurvePresentation runs
updateCurveVisibility, which detaches and deletes m_plotCurve, before
calling updateCurveAppearance. The override in RimWellFlowRateCurve
dereferenced m_plotCurve unconditionally and crashed. Match the null
guard already present in RimPlotCurve::updateCurveAppearance.
Fixes#14011.