When a server is deleted before its floating IP has been disassociated,
the notification doens't contain a `last_known_router_id` key, which
results in a `KeyError` exception being thrown.
This commit gracefully handles this situation by setting
`last_router_id` to `None` when `last_known_router_id` is missing.
Change-Id: If127a33cec7ce6c4d264a191df37c30decab4daa
Closes-Bug: #1795816