Fix NoMethodError when accessing protected environment for job

What does this MR do?

When checking if a job can be :update_build by a user, this user is then passed all the way to ProtectedEnvironment::DeployAccessLevel. As this user can be nil, we return false if so.

Related issue: #30452 (closed)

Screenshots

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.