refactoring (*textRows).readRow in a more clear way

Fix error returns

use utf8mb4 instead of utf8 in TestCharset (#1228)

From MySQL 8.0.24, `SELECT @@character_set_connection` reports utf8mb3 or utf8mb4 instead of utf8.
Because utf8 is currently an alias for utf8mb3, however at some point utf8 is expected to become a reference to utf8mb4.

> ref. https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-24.html#mysqld-8-0-24-bug
> Important Note: When a utf8mb3 collation was specified in a CREATE TABLE statement, SHOW CREATE TABLE, DEFAULT CHARSET,
> the values of system variables containing character set names,
> and the binary log all subsequently displayed the character set as utf8 which is becoming a synonym for utf8mb4.
> Now in such cases, utf8mb3 is shown instead, and CREATE TABLE raises the warning 'collation_name' is a collation of the deprecated character set UTF8MB3.
> Please consider using UTF8MB4 with an appropriate collation instead. (Bug #27225287, Bug #32085357, Bug #32122844)
>
> References: See also: Bug #30624990.

The document says that we should use utf8mb4 instead of utf8, so we should follow it.

driver_test.go

@@ -1450,11 +1450,11 @@ func TestCharset(t *testing.T) {
 	mustSetCharset("charset=ascii", "ascii")
 
 	// when the first charset is invalid, use the second
-	mustSetCharset("charset=none,utf8", "utf8")
+	mustSetCharset("charset=none,utf8mb4", "utf8mb4")
 
 	// when the first charset is valid, use it
-	mustSetCharset("charset=ascii,utf8", "ascii")
-	mustSetCharset("charset=utf8,ascii", "utf8")
+	mustSetCharset("charset=ascii,utf8mb4", "ascii")
+	mustSetCharset("charset=utf8mb4,ascii", "utf8mb4")
 }
 
 func TestFailingCharset(t *testing.T) {

packets.go

@@ -761,40 +761,40 @@ func (rows *textRows) readRow(dest []driver.Value) error {
 	}
 
 	// RowSet Packet
-	var n int
-	var isNull bool
-	pos := 0
+	var (
+		n      int
+		isNull bool
+		pos    int = 0
+	)
 
 	for i := range dest {
 		// Read bytes and convert to string
 		dest[i], isNull, n, err = readLengthEncodedString(data[pos:])
 		pos += n
-		if err == nil {
-			if !isNull {
-				if !mc.parseTime {
-					continue
-				} else {
-					switch rows.rs.columns[i].fieldType {
-					case fieldTypeTimestamp, fieldTypeDateTime,
-						fieldTypeDate, fieldTypeNewDate:
-						dest[i], err = parseDateTime(
-							dest[i].([]byte),
-							mc.cfg.Loc,
-						)
-						if err == nil {
-							continue
-						}
-					default:
-						continue
-					}
-				}
 
-			} else {
-				dest[i] = nil
-				continue
+		if err != nil {
+			return err
+		}
+
+		if isNull {
+			dest[i] = nil
+			continue
+		}
+
+		if !mc.parseTime {
+			continue
+		}
+
+		// Parse time field
+		switch rows.rs.columns[i].fieldType {
+		case fieldTypeTimestamp,
+			fieldTypeDateTime,
+			fieldTypeDate,
+			fieldTypeNewDate:
+			if dest[i], err = parseDateTime(dest[i].([]byte), mc.cfg.Loc); err != nil {
+				return err
 			}
 		}
-		return err // err != nil
 	}
 
 	return nil