Add allow_empty attribute to aws_ami and aws_ebs_snapshot data sources

[joshuaspence]

Fixes #12081.

[joshuaspence]

I'm not really really sure if this is necessary or if there is a better null value.

[joshuaspence]

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWS\(AmiDataSource\|EbsSnapshotDataSource\)_emptyPass' 
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/22 09:10:21 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWS\(AmiDataSource\|EbsSnapshotDataSource\)_emptyPass -timeout 120m
=== RUN   TestAccAWSAmiDataSource_emptyPass
--- PASS: TestAccAWSAmiDataSource_emptyPass (36.18s)
=== RUN   TestAccAWSEbsSnapshotDataSource_emptyPass
--- PASS: TestAccAWSEbsSnapshotDataSource_emptyPass (36.25s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	72.439s

[joshuaspence]

> make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccDataSourceAws\(Ami\|EbsSnapshot\)Ids' 
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/21 07:53:41 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccDataSourceAws\(Ami\|EbsSnapshot\)Ids -timeout 120m
=== RUN   TestAccDataSourceAwsAmiIds_basic
--- PASS: TestAccDataSourceAwsAmiIds_basic (46.85s)
=== RUN   TestAccDataSourceAwsAmiIds_empty
--- PASS: TestAccDataSourceAwsAmiIds_empty (41.59s)
=== RUN   TestAccDataSourceAwsEbsSnapshotIds_basic
--- PASS: TestAccDataSourceAwsEbsSnapshotIds_basic (77.45s)
=== RUN   TestAccDataSourceAwsEbsSnapshotIds_empty
--- PASS: TestAccDataSourceAwsEbsSnapshotIds_empty (35.59s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	201.492s

[joshuaspence]

Actually, let me open a new pull request given that I've changed the implementation a fair bit.

[ghost]

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