return empty list for resource pool without host or parent cluster

This is to address the following error:

Error caught: [NoMethodError] undefined method `collect' for nil:NilClass
vmdb/app/models/resource_pool.rb:165:in `all_host_ids'
vmdb/app/models/mixins/aggregation_mixin.rb:95:in `aggregate_hardware'
vmdb/app/models/mixins/aggregation_mixin.rb:28:in `aggregate_cpu_speed'
...

showing for resource pools, which -- for whatever reason -- do not
have a host or a parent cluster.

https://bugzilla.redhat.com/show_bug.cgi?id=1133569