Allow nullification of `customer_gateway_ip_address` #13

Job	Run time
terraform-module / CI / Labels Validate	3s
terraform-module / Readme	0s
terraform-module / CI / bats	36s
terraform-module / CI / Format HCL	14s
terraform-module / CI / Find modules	3s
terraform-module / CI / Codeowners validate	4s
terraform-module / CI / Lint (.)	14s
terraform-module / CI / Lint (./examples/complete)	20s
terraform-module / ci	1s
terraform-module / Release	0s
	1m 35s

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