Updated the boot network count check to account for no networks in the api request

### Summary
- Adjusted the code to return an empty set when there is no networks field specified in the api request
- Removed the None check in the check_networks function since the function it calls to parse the network no longer returns None and instead returns an empty set

### Testing
- Tested these changes in Rackspace PreProd to validate the requests get rejected properly and still accepts appropriate requests.