Allow to save user details without avatar

Now, when the avatar saved in a different endpoint, we should allow to call
https://api-staging.codingcoach.io/#/%2Fusers/put_users__id_ without avatar field.

Current behaviour: the avatar is saved as empty (which basically deletes the avatar)

Related to https://github.com/Coding-Coach/find-a-mentor/issues/696