Fix enabled_endpoint_uris for object store when endpoint is nil

What does this MR do and why?

ObjectStoreSettings.enabled_endpoint_uris is used in the new gitlab-http gem. When migrating the method, we missed to handle the case when the endpoint is nil.

• Fixes https://new-sentry.gitlab.net/organizations/gitlab/issues/395932/?project=3
• Related to #415686 (closed)
• More info: #426549 (comment 1598185794)

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