Bug: Backend accepts "undefined" string as valid username in `PATCH /users/self` endpoint

[azeemuddinaziz]

Issue Description

Currently in the signup flow at /new-signup, we take firstname and lastname inputs.
After submitting, we hit /username to generate a username. If this fails or errors, the frontend sends a request to update the profile with {"username": "undefined"}.
The backend endpoint PATCH /users/self accepts this body and updates the database with the string "undefined".

The proposed change will:

• Prevent the backend from accepting "undefined" as a username
• Return an error if this invalid data is sent

Expected Behaviour

• The backend should not update the database if username is "undefined"
• The API should return an error for this request

Current Behaviour

• The backend accepts "undefined" string
• The database is updated with "undefined" as the username

Reproducibility

• This issue is reproducible
• This issue is not reproducible

Severity/Priority

• Critical
• High
• Medium
• Low

Additional Information

• This happens during the onboarding flow.
• Endpoint: PATCH /users/self
• Requirement: Instead of a simple check for undefined, fix the root cause.

Screenshot

Checklist

• I have read and followed the project's code of conduct.
• I have searched for similar issues before creating this one.
• I have provided all the necessary information to understand and reproduce the issue.
• I am willing to contribute to the resolution of this issue.