Prevent internal server error if no description passed to API (sync with sciyoshi/redmine-slack commit 5e01d86a12448554ee7015a1ee9df3638bbd7610)

lib/redmine_mattermost/listener.rb

@@ -268,6 +268,10 @@ private
 	end
 
 	def extract_usernames text = ''
+		if text.nil?
+			text = ''
+		end
+
 		# mattermost usernames may only contain lowercase letters, numbers,
 		# dashes and underscores and must start with a letter or number.
 		text.scan(/@[a-z0-9][a-z0-9_\-]*/).uniq