test: add count distinct dictionary nulls

Author: kosiew

datafusion/functions-aggregate/src/count.rs

@@ -711,8 +711,8 @@ impl Accumulator for DistinctCountAccumulator {
         }
 
         (0..arr.len()).try_for_each(|index| {
-            if !arr.is_null(index) {
-                let scalar = ScalarValue::try_from_array(arr, index)?;
+            let scalar = ScalarValue::try_from_array(arr, index)?;
+            if !scalar.is_null() {
                 self.values.insert(scalar);
             }
             Ok(())

datafusion/sqllogictest/test_files/aggregate.slt

@@ -5043,6 +5043,20 @@ select count(distinct column1), count(distinct column2) from dict_test group by
 statement ok
 drop table dict_test;
 
+## count distinct dictionary with null values
+statement ok
+create table dict_null_test as
+    select arrow_cast(NULL, 'Dictionary(Int32, Utf8)') as d
+    from (values (1), (2), (3), (4), (5));
+
+query I
+select count(distinct d) from dict_null_test;
+----
+0
+
+statement ok
+drop table dict_null_test;
+
 # avg_duration
 
 statement ok