Fix request body for compute unshelve action API

Compute unshelve action API accept request body with
action name and null value. For exmaple: {'unshelve': null}.
But Tempest pass the empty dict instead of null- {'unshelve': {}}.
This works fine till 2.77 microversionbut after that nova
started rejecting the empty dict {}.

Defaulting the request body to None so that it works for older
as well newer microversion.

Closes-Bug: #1836187

- https://developer.openstack.org/api-ref/compute/?expanded=unshelve-restore-shelved-server-unshelve-action-detail#unshelve-restore-shelved-server-unshelve-action

Change-Id: I5b4be77d5ae88aedf6fd700e34a293064e655ba8
1 file changed