fix: make "login_name" field optional for API edit user (#38917) (#38945)

Backport #38917

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/swagger/v1_json.tmpl

@@ -25063,7 +25063,6 @@
         "login_name": {
           "description": "identifier of the user, provided by the external authenticator (if configured)",
           "type": "string",
-          "default": "empty",
           "x-go-name": "LoginName"
         },
         "must_change_password": {
@@ -26249,8 +26248,7 @@
       "description": "EditUserOption edit user options",
       "type": "object",
       "required": [
-        "source_id",
-        "login_name"
+        "source_id"
       ],
       "properties": {
         "active": {
@@ -26301,7 +26299,6 @@
         "login_name": {
           "description": "identifier of the user, provided by the external authenticator (if configured)",
           "type": "string",
-          "default": "empty",
           "x-go-name": "LoginName"
         },
         "max_repo_creation": {
@@ -30658,7 +30655,6 @@
         "login_name": {
           "description": "identifier of the user, provided by the external authenticator (if configured)",
           "type": "string",
-          "default": "empty",
           "x-go-name": "LoginName"
         },
         "prohibit_login": {