[#66950180] Allow empty hashes for LoadBalancer services

All parameters underneath the per-service entries for
`virtual_server`->`service_profiles` and `pool`->`service` entries have
defaults, thereby allowing an empty hash as an argument.

The schema prevented this though, which this commit fixes.

Now you can specify:
```
pools:
  name: test-pool
  service:
    http: {}
virtual_servers:
  name: test-vs
  service_profile:
    http: {}
```

NB: This does not fix the 'allow nil' version, which would permit 'http:' vs 'http: {}' -- it's a more involved change, so I thought it best to keep it out of this PR.