sql/distsqlrun: don't inspect encoding output during distinct

In the beginning of times a commit was added to teach DISTINCT about
the difference between filter columns and selected columns. In that
commit a check was added [1] such that the seen marker would only be
added if the encoded version of the column contained > 0 bytes. That
commit doesn't suggest a reason for the addition of that check, and it
is unclear to me now why it was added. (Note that I don't have experience
in the distsql directories, so I may be missing some history.) This file
has been improved since then, but the diligently check remained.

That check caused a GROUP BY with two rows each of empty arrays to not
consider those arrays equal (again, since the seen marker was avoided). A
experiment removing the check showed that no existing tests failed as
a result. And in addition, this new failing test now passed. I can't
find any evidence that this check was necessary, or why it was present
in the first place. I conclude that it is safe to remove until we find
a counter example.

Fixes #37544

[1]: 965107f#diff-6a63b13f6fae0ef7417b27292db3f04aR130

Release note (bug fix): Fix GROUP BY for empty arrays.

pkg/sql/distsqlrun/distinct.go

@@ -246,17 +246,15 @@ func (d *Distinct) Next() (sqlbase.EncDatumRow, *ProducerMetadata) {
 			d.seen = make(map[string]struct{})
 		}
 
-		if len(encoding) > 0 {
-			if _, ok := d.seen[string(encoding)]; ok {
-				continue
-			}
-			s, err := d.arena.AllocBytes(d.Ctx, encoding)
-			if err != nil {
-				d.MoveToDraining(err)
-				break
-			}
-			d.seen[s] = struct{}{}
+		if _, ok := d.seen[string(encoding)]; ok {
+			continue
+		}
+		s, err := d.arena.AllocBytes(d.Ctx, encoding)
+		if err != nil {
+			d.MoveToDraining(err)
+			break
 		}
+		d.seen[s] = struct{}{}
 
 		if outRow := d.ProcessRowHelper(row); outRow != nil {
 			return outRow, nil

pkg/sql/logictest/testdata/logic_test/array

@@ -1280,3 +1280,20 @@ query TT
 SELECT ARRAY[]::int[], ARRAY[]:::int[]
 ----
 {}  {}
+
+subtest 37544
+
+query T
+SELECT
+    col_1
+FROM
+    (
+        VALUES
+            (ARRAY[]::INT8[]),
+            (ARRAY[]::INT8[])
+    )
+        AS tab_1 (col_1)
+GROUP BY
+    tab_1.col_1
+----
+{}