Fix #4614: Counts endpoint responds with invalid JSON.

Caused by the search timing out and returning nil for the count. Nil got
serialized as the empty string instead of as null.

app/controllers/counts_controller.rb

@@ -1,9 +1,15 @@
 class CountsController < ApplicationController
-  respond_to :xml, :json
+  respond_to :html, :xml, :json
 
   def posts
     estimate_count = params.fetch(:estimate_count, "true").truthy?
     skip_cache = params.fetch(:skip_cache, "false").truthy?
     @count = PostQueryBuilder.new(params[:tags], CurrentUser.user).normalized_query.fast_count(timeout: CurrentUser.statement_timeout, estimate_count: estimate_count, skip_cache: skip_cache)
+
+    if request.format.xml?
+      respond_with({ posts: @count }, root: "counts")
+    else
+      respond_with({ counts: { posts: @count }})
+    end
   end
 end