"Validation failed: Last name can't be blank" when trying to use OIDC

## Describe the bug

I'm getting a "Validation failed: Last name can't be blank" exception when trying to use OIDC.

## To Reproduce

1. Configure your OIDC server so that it returns a name without any whitespace
2. Try to log in using OIDC
3. Observe crash

## Expected behaviour

usernames without whitespace are supported.

## Environment details

- OS: all
- Browser: all
- Version: 3.3.4

## Temporary workaround for people facing this issue

change the `oidc.name_field` option to some random value that doesn't exist, doing so will prevent the names from being updated

## Additional information/context

the OIDC spec specifies the `name` field as `End-User's full name in displayable form including all name parts, possibly including titles and suffixes, ordered according to the End-User's locale and preferences.` so we can't assume that it will be in any particular format. And even if we do, the name field is configurable via the `uidc.name_field` option and a user might want to use something like `preferred_username` instead, and that rarely will have any whitespace. The spec also includes `given_name`, `family_name`, and `middle_name` as part of the `Standard Claims` but not every provider supports them. See: https://openid.net/specs/openid-connect-core-1_0.html#rfc.section.5.1

I have 10+ services configured in my OIDC provider and all of them can handle usernames just fine. Imo assuming **anything** about the name field is a recipe for disaster. The spec even mentions `possibly including titles and suffixes, ordered according to the End-User's locale and preferences.`, and people all over the world can have different formats, prefixes, suffixes, middle names, etc.

problematic line: https://github.com/postalserver/postal/blob/da90e75036c27482699921613d838f4058a100e7/app/models/user.rb#L127

trace:
```
[...]
/usr/local/bundle/gems/activerecord-7.0.8.1/lib/active_record/suppressor.rb:54:in `save!'
/opt/postal/app/app/models/user.rb:129:in `find_from_oidc'
/opt/postal/app/app/controllers/sessions_controller.rb:76:in `create_from_oidc'
/usr/local/bundle/gems/actionpack-7.0.8.1/lib/action_controller/metal/basic_implicit_render.rb:6:in `send_action'
[...]
```
log (details redacted with ``):
```
D, [2024-07-21T17:34:00.071792 #10] DEBUG -- omniauth: (oidc) Callback phase initiated.
2024-07-21 17:34:00 +0000 DEBUG got auth details from issuer: # at_hash="" aud=# auth_time=1719762174 azp="postal" client_id="postal" email="nezu@" email_verified=true exp=1721586840 groups=#"]> iat=1721583240 iss="https://auth." jti="" name="nezu" nonce="" preferred_username="nezu" sub="">
2024-07-21 17:34:00 +0000 DEBUG no user with UID for issuer https://auth.
2024-07-21 17:34:00 +0000 DEBUG found user with e-mail address nezu@ (user ID: 1)
2024-07-21 17:34:00 +0000 ERROR GET /auth/oidc/callback?code=&iss=https%3A%2F%2Fauth.&scope=openid+profile+email+groups&state= (422) event=request transaction= controller=SessionsController action=create_from_oidc format=html method=GET path=/auth/oidc/callback?code=&iss=https%3A%2F%2Fauth.&scope=openid+profile+email+groups&state= request_id= ip_address= status=422 db_runtime=4.355344772338867 exception_class=ActiveRecord::RecordInvalid exception_message=Validation failed: Last name can't be blank exception_backtrace=/usr/local/bundle/gems/activerecord-7.0.8.1/lib/active_record/validations.rb:80:in `raise_validation_error'\n/usr/local/bundle/gems/activerecord-7.0.8.1/lib/active_record/validations.rb:53:in `save!'\n/usr/local/bundle/gems/activerecord-7.0.8.1/lib/active_record/transactions.rb:302:in `block in save!'\n/usr/local/bundle/gems/activerecord-7.0.8.1/lib/active_record/transactions.rb:354:in `block in with_transaction_returning_status'
2024-07-21 17:34:00 +0000 Rack app ("GET /auth/oidc/callbackcode=&iss=https%3A%2F%2Fauth.&scope=openid+profile+email+groups&state=" - ()): #
```