Fix cleaning up metasets (#9656)

* Fix cleaning up metasets

I believe it's a mistake to only delete the metaset if it has a valid controller; see f191f2f5 for where this behavior was introduced.

This is a minimal fix for #9653; as discussed there, it may also be worth updating `updateHoverStyle`.

As of #7030, `this._metasets` should always be defined, so checking whether it's undefined is no longer necessary.

* Add a test covering metaset behavior

* Add a regression test for #9653; fix `toHaveSize` usage

* Fix test failure