Allow REST clients to clear an ExpectedMachine BMC IP address

[chet]

The ExpectedMachine REST PATCH should let operators remove a configured bmcIpAddress without replacing the rest of the resource. This gives the field normal PATCH behavior: omission preserves it, an address sets it, and an explicit null clears it.

What this involves

• Preserve whether bmcIpAddress was omitted or explicitly set to null in APIExpectedMachineUpdateRequest.
• Route explicit null through the existing ExpectedMachineDAO.Clear support before forwarding the updated resource through the site workflow to Core. Empty strings remain invalid.
• Apply the same behavior to individual and batch PATCH requests, including mixed batches where omitted values must remain untouched.
• Document the clear behavior in OpenAPI and cover setting, omission, explicit clearing, repeated clearing, the Core workflow payload, and response state.