protect against EmailReplyTrimmer returning nil

lib/gitlab/email/reply_parser.rb

@@ -13,7 +13,7 @@ module Gitlab
         encoding = body.encoding
-        body = EmailReplyTrimmer.trim(body)
+        body = EmailReplyTrimmer.trim(body) or return ""
         # [jneen] not using /\s+$/ here because that deletes empty lines
         body = body.gsub(/[ \t]$/, '')