gatewayId can be null which fails in python

[Uthar]

Deploying a vpcRoute without gatewayId set goes through the nix phase, and fails in the python part. I guess it should be checked for null in the nix phase

e.g.

resources.vpcRoutes.myRoute = {
  region = ...
  routeTableId = ...
  destinationCidrBlock = ..
}
=> UnboundLocalError: local variable 'target' referenced before assignment

resources.vpcRoutes.myRoute = {
  region = ...
  routeTableId = ...
  destinationCidrBlock = ..
  gatewayId = ...
}
=> OK