Guard the remaining unguarded legendConfig() uses so a null legend config
cannot crash: both updateLegend() implementations (eclipse and statistics
projections) now capture the pointer and return early when null, and the
statistics view tree ordering skips a null legend config.
mainGrid(), unionOfActiveCells(), computeUnionOfActiveCells(), and statisticsCaseCollection() default to null/no-op. Remove redundant overrides from RimEclipseCaseEnsemble.