Fix NoMethodError from Projects::CompareController#show

Vasilii Iakliushinrequested to merge
vi-fix-undefined-method-diff-refs into master

What does this MR do and why?

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1790363

Problem

NoMethodError: undefined method `diff_refs' for
nil:NilClass (NoMethodError)

        send_git_diff source_project.repository, compare.diff_refs
                                                        ^^^^^^^^^^

The validation for presence of compare results is missing for patch and diff formats.

Solution

Return 404 error when compare results are not available.

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