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.

spec/lib/scss_checker_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe Stylesheet::Manager::ScssChecker do
-  fab!(:theme) { Fabricate(:theme) }
+  fab!(:theme)
 
   describe "#has_scss" do
     it "should return true when theme has scss" do