Comment 4 for bug 1928623

Based on what's been said here already, it seems like the back-end should return null or an empty list as allowable storage layouts for machines with Allocated status. This will allow disabling the dropdown if the list is empty.

There's a related issue that describes this in more detail: https://github.com/canonical/maas-ui/issues/3258

As a temporary fix this action can be manually disallowed in the UI for allocated machines.