If the plugin does not have a "requires" than the upgrade page gets the wrong string. See screenshot at: https://github.com/moodlerooms/moodle-plugin-ci/issues/59

If it is not required, then the page that displays it has a small bug and should not display anything at the "requires" column.

Otherwise, my suggestion is to update the following page:

https://docs.moodle.org/dev/version.php

and say that $plugin->requires is required.