Fix hstore NULL versus empty

When running queries with the hstore type registered, and with simple
mode queries, the scan implementation does not correctly distinguish
between NULL and empty. Fix the implementation and add a test to
verify this.

pgtype/hstore.go

@@ -174,7 +174,7 @@ func (scanPlanBinaryHstoreToHstoreScanner) Scan(src []byte, dst any) error {
 	scanner := (dst).(HstoreScanner)
 
 	if src == nil {
-		return scanner.ScanHstore(Hstore{})
+		return scanner.ScanHstore(Hstore(nil))
 	}
 
 	rp := 0
@@ -234,7 +234,7 @@ func (scanPlanTextAnyToHstoreScanner) Scan(src []byte, dst any) error {
 	scanner := (dst).(HstoreScanner)
 
 	if src == nil {
-		return scanner.ScanHstore(Hstore{})
+		return scanner.ScanHstore(Hstore(nil))
 	}
 
 	keys, values, err := parseHstore(string(src))

pgtype/hstore_test.go

@@ -2,6 +2,7 @@ package pgtype_test
 
 import (
 	"context"
+	"reflect"
 	"testing"
 
 	"github.com/jackc/pgx/v5"
@@ -179,4 +180,26 @@ func TestHstoreCodec(t *testing.T) {
 	}
 
 	pgxtest.RunValueRoundTripTests(context.Background(), t, ctr, pgxtest.KnownOIDQueryExecModes, "hstore", tests)
+
+	// scan empty and NULL: should be different
+	pgxtest.RunWithQueryExecModes(context.Background(), t, ctr, pgxtest.AllQueryExecModes, func(ctx context.Context, t testing.TB, conn *pgx.Conn) {
+		h := pgtype.Hstore{"should_be_erased": nil}
+		err := conn.QueryRow(ctx, `select cast(null as hstore)`).Scan(&h)
+		if err != nil {
+			t.Fatal(err)
+		}
+		expectedNil := pgtype.Hstore(nil)
+		if !reflect.DeepEqual(h, expectedNil) {
+			t.Errorf("plain conn.Scan failed expectedNil=%#v actual=%#v", expectedNil, h)
+		}
+
+		err = conn.QueryRow(ctx, `select cast('' as hstore)`).Scan(&h)
+		if err != nil {
+			t.Fatal(err)
+		}
+		expectedEmpty := pgtype.Hstore{}
+		if !reflect.DeepEqual(h, expectedEmpty) {
+			t.Errorf("plain conn.Scan failed expectedEmpty=%#v actual=%#v", expectedEmpty, h)
+		}
+	})
 }