Resolve "NoMethodError: undefined method `term_agreements' for nil:NilClass"

What does this MR do?

Fixes #204723 (closed)

Sentry: https://sentry.gitlab.net/gitlab/gitlabcom/issues/1242497/?query=Users::TermsController#accept

A user session is required to accept or decline a term, and this was not being mandated since we are currently applying skip_before_action :authenticate_user! for all actions in this controller.

Skipping this filter is necessary only for the index action, and the accept and decline actions should compulsorily mandate the presence of a logged in user to be able to accept or decline terms.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
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• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
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Security

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• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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