Prevent NPE

[junichi11]

• Do not return null in getOccurrences()
• See Code cleanup for csl.editor.semantic package #8028

@@ -155,7 +146,7 @@ List<OffsetRange> processImpl(ParserResult info, Document doc, int caretPosition
 
         Map<OffsetRange, ColoringAttributes> highlights = finder.getOccurrences();
 
-        return highlights == null ? null : new ArrayList<OffsetRange>(highlights.keySet());
+        return List.copyOf(highlights.keySet());