Should vuln db lastUpdate be optional in vulns predicate

[lumjjb]

As part of vulns 0.2 predicate, there is a vuln db lastUpdate field which is required. This may not necessary be present with some scanners. Some vuln databases only expose the lastUpdate for an entry for a vulnerability and not the version of the entire database (e.g. in OSV), as well as MITRE (to my knowledge).

Thus, i'm wondering if the lastUpdate information should be optional. Following up as discussions from this thread: #392 (comment)