AO3-6795 fix 500 error when excluding search param is invalid

[AliceLoeser]

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• Do you have fewer than 5 pull requests already open? If not, please wait
  until they are reviewed and merged before creating new pull requests.

Issue

https://otwarchive.atlassian.net/browse/AO3-6795

Purpose

This PR fixes a bug that displayed a 500 error when the search is invalid, because @facets is nil since @works is invalid (has an error).

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

          @facets[tag.class.to_s.underscore] ||= []
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  app/controllers/works_controller.rb:123:in `block in index'
    @facets[tag.class.to_s.underscore] ||= []
  app/controllers/works_controller.rb:122:in `index'
    tags.each do |tag|

Testing Instructions

• Go to a tag, e.g. Alternate Universe: https://test.archiveofourown.org/tags/Alternate Universe/works
• In the filters, expand one of the sections for excluding a tag and choose something to exclude, e.g. the Not Rated rating
• Also in the filters, enter something invalid in “Search within results,” e.g. otp: true, sort: words (the comma makes it invalid)
• Press Sort and Filter
• Notice that instead of the 500 error we see an error flash with "Your search failed because of a syntax error" message

Credit

AliceLsr (feminine pronouns)

[Bilka2]

This runs after the filters have been cleared, so this test passes even without the bug fix.

I think it would be good to make a separate scenario for the bad query with an excluded tag, this scenario is already pretty long.