Allow GraphQL UserType avatar_url to be nullable

What does this MR do?

This MR changes the GraphQL UserType#avatar_url field to be nullable.

In the situation where a user has no avatar, and Gravatar is disabled, User#avatar_url can be nil.

Breaking change to UserType#avatar_url

This change constitutes a breaking change to our GraphQL API.

Conversation was raised in #f_graphql (internal, good for 90 days) about the options. See !25491 (comment 294070979) and !25491 (comment 294090173) for the rationale of why we will make this change.

• Issue: #34512 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done