test/builder/amazon/common/step_pre_validate: Update DescribeVpcs mock
test/builder/amazon/common/step_pre_validate: Update DescribeVpcs mock

* Update condition to check for a empty VpcId

ricardclau/packer,timsutton/packer,grubernaut/packer,dave2/packer,timsutton/packer,grubernaut/packer,timsutton/packer,pecigonzalo/packer,dave2/packer,paulmey/packer,timsutton/packer,ricardclau/packer,paulmey/packer,hashicorp/packer,pecigonzalo/packer,dave2/packer,hashicorp/packer,ricardclau/packer,grubernaut/packer,rnaveiras/packer,ricardclau/packer,rnaveiras/packer,paulmey/packer,rnaveiras/packer,timsutton/packer,paulmey/packer,pecigonzalo/packer,dave2/packer,mitchellh/packer,mitchellh/packer,paulmey/packer,mitchellh/packer,grubernaut/packer,ricardclau/packer,grubernaut/packer,dave2/packer,hashicorp/packer,dave2/packer,mitchellh/packer,grubernaut/packer,paulmey/packer,timsutton/packer,rnaveiras/packer,hashicorp/packer,pecigonzalo/packer,mitchellh/packer,ricardclau/packer,rnaveiras/packer,rnaveiras/packer