Fix nulls in aggregations with distinct (#6420)

Closes #6395

Author: mattnibs

runtime/sam/expr/agg/distinct.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 
 	"github.com/brimdata/super"
+	"github.com/brimdata/super/scode"
 )
 
 type distinct struct {
@@ -21,7 +22,7 @@ func newDistinct(f Function) Function {
 
 func (d *distinct) Consume(val super.Value) {
 	d.buf = binary.AppendVarint(d.buf[:0], int64(val.Type().ID()))
-	d.buf = append(d.buf, val.Bytes()...)
+	d.buf = scode.Append(d.buf, val.Bytes())
 	if _, ok := d.seen[string(d.buf)]; ok {
 		return
 	}
@@ -60,7 +61,7 @@ func (d *distinct) Result(sctx *super.Context) super.Value {
 		if err != nil {
 			panic(err)
 		}
-		d.fun.Consume(super.NewValue(typ, bytes))
+		d.fun.Consume(super.NewValue(typ, scode.Bytes(bytes).Body()))
 		delete(d.seen, key)
 	}
 	return d.fun.Result(sctx)

runtime/vam/expr/agg/distinct.go

@@ -28,7 +28,7 @@ func (d *distinct) Consume(vec vector.Any) {
 		b.Truncate()
 		vec.Serialize(&b, i)
 		d.buf = binary.AppendVarint(d.buf[:0], int64(id))
-		d.buf = append(d.buf, b.Bytes().Body()...)
+		d.buf = append(d.buf, b.Bytes()...)
 		if _, ok := d.seen[string(d.buf)]; ok {
 			continue
 		}
@@ -71,7 +71,7 @@ func (d *distinct) Result(sctx *super.Context) super.Value {
 		if err != nil {
 			panic(err)
 		}
-		b.Write(super.NewValue(typ, bytes))
+		b.Write(super.NewValue(typ, scode.Bytes(bytes).Body()))
 		count++
 		if count == 1024 {
 			d.fun.Consume(b.Build())

runtime/ztests/op/aggregate/distinct-null.yaml

@@ -0,0 +1,10 @@
+spq: |
+  avg(distinct this), count(distinct this), collect(distinct this), sum(distinct this)
+
+vector: true
+
+input: |
+  null::float64
+
+output: |
+  {avg:null::float64,count:0::uint64,collect:null,sum:null::float64}