r/aws_cognito_user_pool: fix crash when hashing nil `schema` #14718

Job	Run time
go mod download	36s
markdown-lint	19s
misspell	37s
go build	16s
import-lint	33s
go test	13m 7s
go generate	6m 16s
Sweeper Functions Not Linked	3m 28s
terraform providers schema	17s
tfproviderdocs	11m 55s
	37m 24s

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