Unable to edit custom email provider config

**Describe the bug**
Unable to edit custom email provider config once saved.

https://github.com/authgear/authgear-server/blob/master/portal/src/graphql/portal/SMTPConfigurationScreen.tsx#L344-L352

**Authgear Version**
- Version: [88bc6e8](https://github.com/authgear/authgear-server/commit/88bc6e8f11a176734d8dc99259cd1e91e6b8db5b)

**To Reproduce**
Steps to reproduce the behavior:
1. Enable custom email provider
2. Save
3. Cannot edit

**Expected behavior**
Can edit the config

**Screenshots**
-

**Client Env (if applicable, please complete the following information):**
Any

**Additional context**
-

Start by reading portal/src/graphql/portal/SMTPConfigurationScreen.tsx around lines 344-352 and reproduce the save-then-edit flow with a custom email provider enabled. Trace why the saved configuration cannot be edited, then verify that the existing configuration can be changed and saved again.

Written by the indexing model from the issue text.