fix(workspacesView): workaround for undefined `_workspaces`

[mmstick]

Related to https://github.com/pop-os/pop/issues/2893

There seems to be a possible scenario where the workspace overview code is run at a time when the _workspaces property lacks any values, or perhaps is undefined itself. Whether it's caused by an extension or something else, we can add a check for undefined-ness and use a height of 0 by default if no workspace is found.

It would be ideal if @magicalraccoon could test this possible fix.