Login and CurrentUser queries yield broken responses

[sdobbelaere]

When doing a login with an incorrect username/password or querying me/current_user both cases result in a cryptical reponse:

  "data": null,
  "errors": [
    {
      "message": "Cannot return null for non-nullable field Query.me.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "me"
      ]
    }
  ]
}

This PR corrects this behaviour by raising an error and with an understandable error.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Documentation

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (c7f4643) 87.27% compared to head (0591a34) 87.27%.

Files	Patch %	Lines
strawberry_django/auth/mutations.py	53.84%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #421   +/-   ##
=======================================
  Coverage   87.27%   87.27%           
=======================================
  Files          37       37           
  Lines        3119     3121    +2     
=======================================
+ Hits         2722     2724    +2     
  Misses        397      397           

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[bellini666]

This looks good in general :)
Some minor changes and we should be good to merge

[bellini666]

Thank you! :)