Fix NoMethodError for OrganizationClusterAgentsFinder

What does this MR do and why?

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1754306

NoMethodError: undefined method `can?' for nil:NilClass (NoMethodError)

      return Clusters::Agent.none unless organization && user.can?(:read_organization_cluster_agent_mapping,
                                                             ^^^^^
  from remote_development/organization_cluster_agents_finder.rb:10:in `execute'
  from resolvers/remote_development/organization/cluster_agents_resolver.rb:22:in `resolve'

Problem

User object can be nil when GraphQL endpoint is used without an authentication.

Solution

Use Ability.allowed? that can handle empty users.

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