Adding issues to a list where the board is scoped to `Any`, `None`, or `0` weights returns 422 (Unprocessable Entity)

Summary

Adding issues to a list where the board is scoped to Any, None, or 0 weights returns 422 (Unprocessable Entity), and the issue isn't created.

By default, it seems boards are created with a weight value of null (unless one of those weights is specified). Any of the above scopes become -1, which may explain the behavior in https://gitlab.com/gitlab-org/gitlab-ce/issues/50008.

ZD: https://gitlab.zendesk.com/agent/tickets/101574 (internal)

/cc @felipe_artur

Steps to reproduce

  1. Create a board scoped to either of the above weights
  2. Add an issue to a list

Example Project

https://gitlab.com/services-support-testing/d/board/boards/696438?=

What is the current bug behavior?

422 error returned in the console

What is the expected correct behavior?

The issue should be added

Relevant logs and/or screenshots

{"method":"POST","path":"<omitted>","format":"json","controller":"Boards::IssuesController","action":"create","status":422,"duration":170.38,"view":0.24,"db":92.67,"time":"2018-08-07T20:54:04.600Z","remote_ip":"<omitted>","user_id":<omitted>,"username":"<omitted>","response":"{\"weight\":[\"must be greater than or equal to 0\"]}","tag":"rails.production","environment":"prd","hostname":"web-01","fqdn":"web-01.sv.prd.gitlab.com","message":null}

Kibana (internal)

Output of checks

This bug happens on GitLab.com (11.2.0-rc2-ee)