Update lastModified For Stacks/Samples Without Versions

[Jdubrick]

Which area is this issue related to?

/area registry

Issue Description

After completion of https://github.com/devfile/api/issues/1327 investigation will need to be done for handling stacks or samples that do not have a version attached to them. This implementation will most likely need to be on the index/generator side and may require fetching the date from a remote location. In addition to the investigation of this issue the implementation should also be contained within this issue as it shouldn't be a large fix.

Acceptance Criteria

• [ ] Stacks or samples without version field no longer display the default time.Time value for lastModified field.

[Jdubrick]

Related issue: https://github.com/devfile/api/issues/1327