Fix invalid DSN after dsnAddTimeout and "" DSN

plugins/inputs/mysql/mysql.go

@@ -216,6 +216,12 @@ func (m *Mysql) gatherServer(serv string, acc telegraf.Accumulator) error {
 }
 
 func dsnAddTimeout(dsn string) (string, error) {
+
+	// DSN "?timeout=5s" is not valid, but "/?timeout=5s" is valid ("" and "/"
+	// are the same DSN)
+	if dsn == "" {
+		dsn = "/"
+	}
 	u, err := url.Parse(dsn)
 	if err != nil {
 		return "", err

plugins/inputs/mysql/mysql_test.go

@@ -92,11 +92,7 @@ func TestMysqlDNSAddTimeout(t *testing.T) {
 	}{
 		{
 			"",
-			"?timeout=5s",
-		},
-		{
-			"127.0.0.1",
-			"127.0.0.1?timeout=5s",
+			"/?timeout=5",
 		},
 		{
 			"tcp(192.168.1.1:3306)/",