Fix nil coercion updating storage size on project statistics

What does this MR do?

With https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25321#note_179632487 we introduced a nullable column involved in ProjectStatistics#update_storage_size until the background migration is completed this column may be nil failing all the updates on Project#statistics.

For details see https://gitlab.com/gitlab-org/gitlab-ce/issues/63020#note_179663636

This MR aims to reach production as soon as possible in order to restore it, data recovery will be handled into another MR.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Related to #63020 (closed)