DEV: Allow fab! without block (#24314)

The most common thing that we do with fab! is:

    fab!(:thing) { Fabricate(:thing) }

This commit adds a shorthand for this which is just simply:

    fab!(:thing)

i.e. If you omit the block, then, by default, you'll get a `Fabricate`d object using the fabricator of the same name.

plugins/chat/spec/integration/outgoing_web_hooks_spec.rb

@@ -99,7 +99,7 @@ RSpec.describe "Outgoing chat webhooks" do
     end
 
     context "for a category channel" do
-      fab!(:category) { Fabricate(:category) }
+      fab!(:category)
       fab!(:chat_channel) { Fabricate(:category_channel, chatable: category) }
       fab!(:chat_message) { Fabricate(:chat_message, chat_channel: chat_channel, user: user1) }