Error 500 when requesting LDAP API endpoint when no LDAP providers configured

https://sentry.gitlap.com/gitlab/gitlabcom/issues/114229/

NoMethodError: undefined method `[]' for nil:NilClass
  lib/api/ldap.rb:24:in `block (2 levels) in <class:Ldap>'
    provider = Gitlab::LDAP::Config.available_servers.first['provider_name']
  lib/gitlab/middleware/multipart.rb:93:in `call'
    return @app.call(env) if encoded_message.blank?
  lib/gitlab/request_profiler/middleware.rb:14:in `call'
    @app.call(env)
  lib/gitlab/database/load_balancing/rack_middleware.rb:37:in `call'
    result = @app.call(env)
  lib/gitlab/jira/middleware.rb:15:in `call'
    @app.call(env)
...
(114 additional frame(s) were not displayed)

NoMethodError: undefined method `[]' for nil:NilClass