fix: fail fast when RspecApiDocumentation.configuration.app is null

lib/rspec_api_documentation/rack_test_client.rb

@@ -61,7 +61,7 @@ def rack_test_session
         rescue LoadError
           raise "#{self.class.name} requires Rack::Test >= 0.5.5. Please add it to your test dependencies."
         end
-      end.new(app)
+      end.new(app || fail("RspecApiDocumentation.configuration.app is nil"))
     end
   end
 end