fix NPE (#16897)

processing/src/main/java/org/apache/druid/query/rowsandcols/ArrayListRowsAndColumns.java

@@ -144,6 +144,9 @@ public class ArrayListRowsAndColumns<RowType> implements AppendableRowsAndColumn
   {
     if (!rowSignature.contains(name)) {
       final Column retVal = extraColumns.get(name);
+      if (retVal == null) {
+        return null;
+      }
       if (numRows() == rows.size()) {
         return retVal;
       }

sql/src/test/resources/calcite/tests/window/arrayConcatAgg.sqlTest

@@ -0,0 +1,13 @@
+type: "operatorValidation"
+
+sql: |
+  SELECT
+    cityName,
+    array_concat_agg(ARRAY[cityName], 10000) over (PARTITION BY cityName)
+  FROM wikipedia
+  WHERE countryName='Austria' and cityName is not null
+  GROUP BY cityName
+
+expectedResults:
+  - ["Horsching","[\"Horsching\"]"]
+  - ["Vienna","[\"Vienna\"]"]