resource/aws_pinpoint_app: Fix panic when nested objects are empty #10643

Job	Run time
go mod download	20s
markdown-lint	20s
misspell	28s
go build	1m 39s
go test	13m 18s
go generate	4m 53s
import-lint	30s
terraform providers schema	29s
Sweeper Functions Not Linked	3m 18s
tfproviderdocs	11m 7s
	36m 22s

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