Properly persist "-1" and "null" for board weight

What does this MR do?

Modify the behaviour of weight selection via Edit Board modal. Persist null for no filter (any), and -1 for no weight.

Use Any to specifically mean No filter. Any not-null value (worded better) can be added when we iterate on this.

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30609

Note: milestone MR is here -> https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14586

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #4221 (closed) and #11529 (closed)