fix: fixed getting a users' name on UserData collections

[NiedielnitsevIvan]

Description

This PR is addressed at fixing the incorrect saving of the user full name in the credentials service.
In the integration between edx-platform and the credentials service, the username is not passed to credentials correctly.
Since in OeX the full name of the user specified in the registration form is stored in the profile, the standard method of the User class get_full_name will not work correctly, because it interacts with user.first_name and user.last_name, which are empty.

Testing instructions

• Create a user on the OeX platform;
• Complete the course and get a certificate;
• Go to the admin panel of the credentials service;
• Check the correctness of the new user's full name.

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Thanks for the pull request, @NiedielnitsevIvan!

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