Make GroupDeployKeys fingerprint nullable

Problem

FIPS-enforcing environments don't support MD5 algorithm. Our plan is to deprecate MD5 fingerprints in the future.

Table group_deploy_keys was created from table keys. But there is a difference between them. group_deploy_keys requires MD5 fingerprint value presence but keys table does not.

Proposal

Add support for NULL values to fingerprint field from `group_deploy_keys. It should not have any effect at the moment but will simplify future steps of MD5 deprecation process.