Statement.getFeatures() throws NullPointerException on plain SELECT DISTINCT #2573

Statement.getFeatures() throws a NullPointerException for a plain SELECT DISTINCT, so a read-only guard that calls it has to fail closed on every DISTINCT query.

Version: com.manticore-projects.jsqlformatter:jsqlparser:5.4.351-SNAPSHOT (build 5.4.351-20260910.100601-1), master at b2115ac8.

Reproduce

CCJSqlParserUtil.parse("SELECT DISTINCT id FROM foo").getFeatures();

java.lang.NullPointerException: Cannot invoke "java.util.List.iterator()" because the return value of "net.sf.jsqlparser.statement.select.Distinct.getOnSelectItems()" is null
      at net.sf.jsqlparser.statement.select.SelectVisitorAdapter.visit(SelectVisitorAdapter.java:149)
      at net.sf.jsqlparser.statement.StatementFeatureVisitor$FeatureSelectVisitor.visit(StatementFeatureVisitor.java:801)
      at net.sf.jsqlparser.statement.StatementFeatureVisitor$FeatureSelectVisitor.visit(StatementFeatureVisitor.java:769)

SELECT DISTINCT ON (id) id FROM foo and SELECT ALL id FROM foo classify fine; only a Distinct without an ON list is affected.

Cause

SelectVisitorAdapter.visit(PlainSelect) (SelectVisitorAdapter.java:149) iterates plainSelect.getDistinct().getOnSelectItems() without a null check; Distinct.getOnSelectItems() is null unless DISTINCT ON (..) was written. SelectDeParser (:531) guards the same list with != null.

Suggestion

Guard the loop in SelectVisitorAdapter (if (onSelectItems != null)), and add SELECT DISTINCT a FROM t to StatementFeatureVisitorTest / AdapterRegressionTest. Setting an empty list from the outside is not a workaround: the deparser then prints DISTINCT ON ().