Check if RSpec::Mocks is defined in #skip_signature_verification?

Trying to respect this comment relating to requiring less of RSpec mocks:
https://github.com/rspec/rspec-rails/blob/1ba134da2c8a48c3e4a26d6bb9104ef33d91781f/lib/rspec/rails/matchers/have_enqueued_mail.rb#L1-L3

Author: odlp

lib/rspec/rails/matchers/active_job.rb

@@ -192,6 +192,8 @@ def detect_args_signature_mismatch(jobs)
           end
 
           def skip_signature_verification?
+            return true unless defined?(RSpec::Mocks)
+
             !RSpec::Mocks.configuration.verify_partial_doubles? ||
               RSpec::Mocks.configuration.temporarily_suppress_partial_double_verification
           end