Handle and test different `link_types` in Resolvers::Vulnerabilities::IssueLinksResolver

Currently the vulnerability issue link resolver defaults to a nil value but then calls to_s.downcase. 🤔

We should test for a GraphQL error if the link_type isn't one we expect.

We can use a ready? method in the resolver for that.

Implementation plan

• backend Add ready? method to Resolvers::Vulnerabilities::IssueLinksResolver (https://gitlab.com/gitlab-org/gitlab/blob/30b6f732090b2307bb4673e9bac45fbe2a9207e0/ee/app/graphql/resolvers/vulnerabilities/issue_links_resolver.rb#L5) to verify if provided link_type is one of ::Vulnerabilities::IssueLink.link_types, of not respond with error (https://docs.gitlab.com/ee/development/api_graphql_styleguide.html#correct-use-of-resolverready)