NoMethodError: undefined method `length' for nil:NilClass

Looks like when https://gitlab.com/search/autocomplete is hit with no search term, a 500 error is thrown

Sentry Issue: GITLABCOM-11Q9

NoMethodError: undefined method `length' for nil:NilClass
  app/helpers/search_helper.rb:40:in `resource_results'
    return [] if term.length < Gitlab::Search::Params::MIN_TERM_LENGTH
  app/helpers/search_helper.rb:31:in `search_autocomplete_opts'
    resource_results(term),
  app/controllers/search_controller.rb:124:in `autocomplete'
    render json: Gitlab::Json.dump(search_autocomplete_opts(term, filter: @filter, scope: @scope))
  app/controllers/application_controller.rb:549:in `block in allow_gitaly_ref_name_caching'
    yield
  lib/gitlab/gitaly_client.rb:352:in `allow_ref_name_caching'
    yield
...
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