Fix an error with empty NAT IPs

When deploying an instance having a NAT IP passed as 'nat_ip: null' in the deployment template, the request arrives at GCP plugin with 'nat_ip: ""' (null value is replaced with an empty string). This leads to deployment failure with error "The specified external IP address '' was not found in region ...".

This commit workarounds the error by patching NAT IP values in the request body to have value None instead of empty string.