Fix NULL being lost when scanning unknown OID into sql.Scanner

https://github.com/jackc/pgx/issues/1078

pgtype.go

@@ -588,7 +588,11 @@ type scanPlanSQLScanner struct{}
 
 func (scanPlanSQLScanner) Scan(ci *ConnInfo, oid uint32, formatCode int16, src []byte, dst interface{}) error {
 	scanner := dst.(sql.Scanner)
-	if formatCode == BinaryFormatCode {
+	if src == nil {
+		// This is necessary because interface value []byte:nil does not equal nil:nil for the binary format path and the
+		// text format path would be converted to empty string.
+		return scanner.Scan(nil)
+	} else if formatCode == BinaryFormatCode {
 		return scanner.Scan(src)
 	} else {
 		return scanner.Scan(string(src))

pgtype_test.go

@@ -2,6 +2,7 @@ package pgtype_test
 
 import (
 	"bytes"
+	"database/sql"
 	"errors"
 	"net"
 	"testing"
@@ -211,6 +212,16 @@ func TestConnInfoScanUnknownOIDTextFormat(t *testing.T) {
 	assert.EqualValues(t, 123, n)
 }
 
+func TestConnInfoScanUnknownOIDIntoSQLScanner(t *testing.T) {
+	ci := pgtype.NewConnInfo()
+
+	var s sql.NullString
+	err := ci.Scan(0, pgx.TextFormatCode, []byte(nil), &s)
+	assert.NoError(t, err)
+	assert.Equal(t, "", s.String)
+	assert.False(t, s.Valid)
+}
+
 func BenchmarkConnInfoScanInt4IntoBinaryDecoder(b *testing.B) {
 	ci := pgtype.NewConnInfo()
 	src := []byte{0, 0, 0, 42}