Fix error when creating issues in scoped boards

What does this MR do?

Fix: Trying to create issues with invalid weights through scoped issue boards

This merge request fixes the problem when board is scoped to none or any weight. The problem
about when weight is zero should be fixed in https://gitlab.com/gitlab-org/gitlab-ee/issues/11529.

For more information check https://gitlab.com/gitlab-org/gitlab-ee/issues/7177#note_167698025

Related to: #7177 (closed)

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