Add null test (#11760)

Author: Dandandan

datafusion/sqllogictest/test_files/aggregate.slt

@@ -1243,6 +1243,12 @@ SELECT APPROX_PERCENTILE_CONT(v, 0.5) FROM (VALUES (1), (2), (3), (NULL), (NULL)
 ----
 2
 
+# percentile_cont_with_nulls_only
+query I
+SELECT APPROX_PERCENTILE_CONT(v, 0.5) FROM (VALUES (CAST(NULL as INT))) as t (v);
+----
+NULL
+
 # csv_query_cube_avg
 query TIR
 SELECT c1, c2, AVG(c3) FROM aggregate_test_100 GROUP BY CUBE (c1, c2) ORDER BY c1, c2