DEV: Fix deprecations in specs (#237)

Author: cvx

spec/requests/docs_controller_spec.rb

@@ -313,15 +313,15 @@ def get_tags_from_response(response_tags)
       end
 
       it "should return a docs topic when only tags are added to settings" do
-        SiteSetting.docs_categories = nil
+        SiteSetting.docs_categories = ""
 
         get "/#{GlobalSetting.docs_path}.json?topic=#{topic.id}"
 
         expect(response.parsed_body["topic"]["id"]).to eq(topic.id)
       end
 
       it "should return a docs topic when only categories are added to settings" do
-        SiteSetting.docs_tags = nil
+        SiteSetting.docs_tags = ""
 
         get "/#{GlobalSetting.docs_path}.json?topic=#{topic.id}"