In `IkiWiki::preprocess`, the last capturing group in the regex used to parse directives in prefix_directives mode is of the form `(\s+...)?\]\]`, which will not be matched if the directive is something without arguments or whitespace, like `\[[!orphans]]`. As a result, its value is undef instead of being an empty string, causing a warning when it is used in the anonymous sub `$handle`. A trivial fix is to treat it as "" if it is undef.
[[patch]] in the master branch of my git repository, and quoted here. --[[smcv]]