Several pieces of code tested only for the presence of keys in `focusFilters` to
determine whether a focus filter was configured, rather than whether the
property was set to a "valid" value for that particular filter. The
`changeFocusModeUser` store action would always set this property, instead of
removing it when the passed user info was empty (logically meaning "no user is
focused").

Fixes https://github.com/hypothesis/client/issues/6109