Fix 500 error for Group#web_url method if nested group

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/2772

No changelog needed because users are not affected by it yet.

cc @smcgivern

enabled an automatic merge when the pipeline for c45ef9fc succeeds

added 1 commit

• 0d9e80c3 - Fix 500 error for Group#web_url method if nested group

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enabled an automatic merge when the pipeline for 0d9e80c3 succeeds

merged

mentioned in commit 4480bef1