Fix panic when parsing empty pgsql host (#28708)

Regression of #27723
Fix #28705

Author: wxiaoguang

modules/setting/database.go

@@ -168,7 +168,7 @@ func getPostgreSQLConnectionString(dbHost, dbUser, dbPasswd, dbName, dbsslMode s
 		RawQuery: dbParam,
 	}
 	query := connURL.Query()
-	if dbHost[0] == '/' { // looks like a unix socket
+	if strings.HasPrefix(dbHost, "/") { // looks like a unix socket
 		query.Add("host", dbHost)
 		connURL.Host = ":" + port
 	}

modules/setting/database_test.go

@@ -65,6 +65,10 @@ func Test_getPostgreSQLConnectionString(t *testing.T) {
 		SSLMode string
 		Output  string
 	}{
+		{
+			Host:   "", // empty means default
+			Output: "postgres://:@127.0.0.1:5432?sslmode=",
+		},
 		{
 			Host:    "/tmp/pg.sock",
 			User:    "testuser",