issues: make issue_prefix optional again Commit 39a59e6915bb398b42c3c2a63c48a950e9d63b55 (helpers: refactor and optimize urlify_issues) made issue_prefix mandatory, while previously it could be empty. An empty issue_prefix is useful when the entire issue pattern needs to be used in the created link. For example, consider a pattern 'PR123' that needs to be translated into: http://example.com/pullrequests/PR123. This could be configured with: issue_pat = (PR\d+) issue_server_link = http://example.com/pullrequests/{id} issue_prefix = We still refuse the issue pattern when issue_prefix is not present at all.