Fix ResultReader.Read() to handle nil values

The ResultReader.Read() method was erroneously converting nil values
to []byte{}.

#1987

pgconn/pgconn.go

@@ -1515,8 +1515,10 @@ func (rr *ResultReader) Read() *Result {
 		values := rr.Values()
 		row := make([][]byte, len(values))
 		for i := range row {
-			row[i] = make([]byte, len(values[i]))
-			copy(row[i], values[i])
+			if values[i] != nil {
+				row[i] = make([]byte, len(values[i]))
+				copy(row[i], values[i])
+			}
 		}
 		br.Rows = append(br.Rows, row)
 	}

pgconn/pgconn_test.go

@@ -1176,6 +1176,24 @@ func TestResultReaderValuesHaveSameCapacityAsLength(t *testing.T) {
 	ensureConnValid(t, pgConn)
 }
 
+// https://github.com/jackc/pgx/issues/1987
+func TestResultReaderReadNil(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
+	defer cancel()
+
+	pgConn, err := pgconn.Connect(ctx, os.Getenv("PGX_TEST_DATABASE"))
+	require.NoError(t, err)
+	defer closeConn(t, pgConn)
+
+	result := pgConn.ExecParams(ctx, "select null::text", nil, nil, nil, nil).Read()
+	require.NoError(t, result.Err)
+	require.Nil(t, result.Rows[0][0])
+
+	ensureConnValid(t, pgConn)
+}
+
 func TestConnExecPrepared(t *testing.T) {
 	t.Parallel()