Currently, any registered stage is printed in the list produced by `$listMqlEntities`. This includes extension stages that are loaded into the server.

However, testing functionality in null_chars_string_input.js derives from `$listMqlEntities` by using the list of stages to ensure that they all are tested against null character input. Because `$listMqlEntities` lists extensions and these aren't tested in this file, the test fails due to a lack of coverage.

This ticket should investigate a solution to this problem better than the current one of manually adding each test extension to the list of `skips` in the file.