Don't use empty key when checking if cipher is usable

Closed username-removed-1345292 requested to merge civz/cryptsetup:fips-xts into master

Since kernel commit 28856a9e52c7cac712af6c143de04766617535dc (, aes-xts-plain64 cipher is unusable in FIPS mode. FIPS now requires AES XTS key to be composed of two non-equal parts. The zero-filled empty_key as used in LUKS_check_cipher() doesn't match the new criteria and is rejected by the kernel, resulting in a failed cipher check. Use a different dummy key.