Use external pattern when mentionable does not belong to project

In commit 020295ff a regex was added to quickly determine if mentionables might
be referenced. But this didn't take into account the mentionable might not
belong to a project. So when it doesn't belong to a project, use the
EXTERNAL_PATTERN to search if there are mentionables.

Fixes gitlab-org/gitlab-ce#31993

app/models/concerns/mentionable.rb

@@ -92,7 +92,7 @@ module Mentionable
   # Uses regex to quickly determine if mentionables might be referenced
   # Allows heavy processing to be skipped
   def matches_cross_reference_regex?
-    reference_pattern = if project.default_issues_tracker?
+    reference_pattern = if project && project.default_issues_tracker?
                           ReferenceRegexes::DEFAULT_PATTERN
                         else
                           ReferenceRegexes::EXTERNAL_PATTERN