ci(danger): prevent a roulette spin if no user found

What does this MR do?

Danger failed in https://gitlab.com/gitlab-org/gitlab-ui/-/jobs/554138550 because the username associated with the component:chart label for UX reviews could not be matched with an actual user in the roulette's data.

This MR removes nil entries form the UX reviewers array before attempting a roulette spin, effectively preventing such error from occurring.

Danger report example with this fix

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.