Sorting by weight causes a 500 error

Can be reproduced by going to Merge Requests and sorting either by "Less weight" or "More weight"

Demonstrable here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests?scope=all&sort=weight_desc&state=opened

Somewhere between RC1 and RC3 this problem was introduced. See the spike in issues since RC3 was deployed to GitLab: https://sentry.gitlap.com/gitlab/gitlabcom/issues/7040/

MRs can't be assigned weight, so that's presumably why this happens. A test should also be written to prevent this in the future.