It's in the main.php of your active template /dokuwiki/lib/tpl/yourtemplate/main.php
.
The code looks something like this:
https://codesearch.dokuwiki.org/xref/dokuwiki/lib/tpl/dokuwiki/main.php#20
It tries to put the title of your wiki in those brackets, which it gets from the config. If you have not configured a wiki title, the brackets remain empty.
I changed this line to:
<title><?php echo strip_tags($conf['title']) ?> - <?php tpl_pagetitle() ?></title>