ec2/customer_gateway: Remove empty validation

[YakDriver]

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Relates #13943

Output from acceptance testing:

% make testacc TESTS=TestAccEC2CustomerGateway PKG=ec2
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/ec2/... -v -count 1 -parallel 20 -run='TestAccEC2CustomerGateway'  -timeout 180m
--- PASS: TestAccEC2CustomerGateway_disappears (27.07s)
--- PASS: TestAccEC2CustomerGatewayDataSource_id (31.32s)
--- PASS: TestAccEC2CustomerGatewayDataSource_filter (31.36s)
--- PASS: TestAccEC2CustomerGateway_basic (31.73s)
--- PASS: TestAccEC2CustomerGateway_deviceName (31.94s)
--- PASS: TestAccEC2CustomerGateway_4ByteASN (32.40s)
--- PASS: TestAccEC2CustomerGateway_tags (56.20s)
--- PASS: TestAccEC2CustomerGateway_certificate (121.22s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/ec2	122.712s

[ewbankkit]

LGTM 🚀.

% make testacc TESTS=TestAccEC2CustomerGateway_ PKG=ec2
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/ec2/... -v -count 1 -parallel 20 -run='TestAccEC2CustomerGateway_'  -timeout 180m
=== RUN   TestAccEC2CustomerGateway_basic
=== PAUSE TestAccEC2CustomerGateway_basic
=== RUN   TestAccEC2CustomerGateway_disappears
=== PAUSE TestAccEC2CustomerGateway_disappears
=== RUN   TestAccEC2CustomerGateway_tags
=== PAUSE TestAccEC2CustomerGateway_tags
=== RUN   TestAccEC2CustomerGateway_deviceName
=== PAUSE TestAccEC2CustomerGateway_deviceName
=== RUN   TestAccEC2CustomerGateway_4ByteASN
=== PAUSE TestAccEC2CustomerGateway_4ByteASN
=== RUN   TestAccEC2CustomerGateway_certificate
=== PAUSE TestAccEC2CustomerGateway_certificate
=== CONT  TestAccEC2CustomerGateway_basic
=== CONT  TestAccEC2CustomerGateway_deviceName
=== CONT  TestAccEC2CustomerGateway_tags
=== CONT  TestAccEC2CustomerGateway_4ByteASN
=== CONT  TestAccEC2CustomerGateway_disappears
=== CONT  TestAccEC2CustomerGateway_certificate
--- PASS: TestAccEC2CustomerGateway_disappears (31.16s)
--- PASS: TestAccEC2CustomerGateway_deviceName (37.09s)
--- PASS: TestAccEC2CustomerGateway_basic (37.32s)
--- PASS: TestAccEC2CustomerGateway_4ByteASN (37.35s)
--- PASS: TestAccEC2CustomerGateway_tags (68.26s)
--- PASS: TestAccEC2CustomerGateway_certificate (118.66s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/ec2	123.002s

[github-actions]

This functionality has been released in v4.0.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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