Fix create-env bug where an empty slice of disks from the state file results in null being passed to the CPI ask the disks argument

[jpalermo]

This is caused by the way the original string slice is created. If you create it using var []string, it json marshals as null Instead we need to make([]string, 0), which will json marshal as []

[jpalermo]

Error seen using the vsphere cpi before this change:

Deploying:
  Creating instance 'bosh/0':
    Creating VM:
      Creating vm with stemcell cid 'sc-ca101912-4a4c-434a-9e7a-5ce2e426f6a4':
        CPI 'create_vm' method responded with error: CmdError{"type":"Unknown","message":"undefined method `map' for nil","ok_to_retry":false}

Exit code 1