Add RSpec/EmptyOutput rubocop rule

This rule was added in rubocop-rspec 2.29.0. While we never use the
`output` matcher, it might actually be a good idea to use it in tests of
take tasks, to check the logger output.

.rubocop.yml

@@ -565,6 +565,9 @@ RSpec/EmptyLineAfterExampleGroup:
   Exclude:
     - spec/factories/**/*
 
+RSpec/EmptyOutput:
+  Enabled: true
+
 RSpec/ExampleWording:
   Enabled: true