BUG: Fix handling empty migration list for VMs

[DavidFair]

Description:

If a VM had an empty migration list we would die if we asked nova faster than the migration turned up. Instead handle this case and use the existing wait for migration code to retry

Special Notes:

Submitter:

Have you (where applicable):

• Added unit tests?
• Checked the latest commit runs on Dev?
• Updated the example config file(s) and README?

---

Reviewer

Does this PR:

• Place non-StackStorm code into the lib directory?
• Have unit tests for the action/sensor and lib layers?
• Have clear and obvious action parameter names and descriptions?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.19%. Comparing base (bdf2fa7) to head (c678935).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #464   +/-   ##
=======================================
  Coverage   99.19%   99.19%           
=======================================
  Files         112      112           
  Lines        2737     2742    +5     
  Branches      339      339           
=======================================
+ Hits         2715     2720    +5     
  Misses         19       19           
  Partials        3        3           

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[DavidFair]

Testing on dev with a fresh VM:

    2026-07-01 14:49:38,805 - apis.openstack_api.openstack_server - INFO - Migrating server: dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138
    2026-07-01 14:49:39,442 - apis.openstack_api.openstack_server - INFO - No migration details available for dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138 yet
    2026-07-01 14:49:44,469 - apis.openstack_api.openstack_server - INFO - Status of migration of server dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138: migrating
    2026-07-01 14:49:49,491 - apis.openstack_api.openstack_server - INFO - Status of migration of server dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138: migrating
    2026-07-01 14:49:54,515 - apis.openstack_api.openstack_server - INFO - Status of migration of server dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138: post-migrating
    2026-07-01 14:49:59,543 - apis.openstack_api.openstack_server - INFO - Status of migration of server dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138: post-migrating
    2026-07-01 14:50:04,570 - apis.openstack_api.openstack_server - INFO - Status of migration of server dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138: post-migrating
    2026-07-01 14:50:09,594 - apis.openstack_api.openstack_server - INFO - Status of migration of server dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138: post-migrating
    2026-07-01 14:50:14,617 - apis.openstack_api.openstack_server - INFO - Status of migration of server dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138: finished
    2026-07-01 14:50:14,693 - apis.openstack_api.openstack_server - INFO - Confirming resize for dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138
    2026-07-01 14:50:14,740 - apis.openstack_api.openstack_server - INFO - Migration completed of server: dfc7ba5a-e8a8-4fc3-a3f3-4e0fbc277138
    '
  stdout: ''