Ensure the module still work with the ESXi where CustomFieldsManager
does not exist.
From: https://www.vmware.com/support/developer/converter-sdk/conv60_apireference/vim.CustomFieldsManager.html
The CustomFieldsManager object is used to add and remove custom fields to
managed entities.
The custom fields values set on managed entities are available through the
customValue property and through the summary objects for VirtualMachine
and HostSystem. They are not available directly through this managed object.
This functionality is only available through VirtualCenter.
Fixes: #56071
cf78759f5b