Fixes #38128 - allow blank image mode digests

[ianballou]

What are the changes introduced in this pull request?

Allows blank digests to support the format of bootc internals publish-rhsm-facts:

{
  "bootc.booted.image": "quay.io/centos-bootc/centos-bootc:stream10",
  "bootc.booted.version": "stream10.20241230.0",
  "bootc.booted.digest": "sha256:48884b0220bb075ce50ae31d8e6739f3e9b3870e746a4b3c87ea2fe3353b7208",
  "bootc.staged.image": "",
  "bootc.staged.version": "",
  "bootc.staged.digest": "",
  "bootc.rollback.image": "quay.io/centos-bootc/centos-bootc:stream10",
  "bootc.rollback.version": "stream10.20241216.0",
  "bootc.rollback.digest": "sha256:e168d2f013ed4cc89771d2c5d1b15951ae09792ed3d647226f48e5b62316d7fb",
  "bootc.available.image": "",
  "bootc.available.version": "",
  "bootc.available.digest": ""
}

Considerations taken when implementing this change?

Our UI appears to be checking for empty string properly as opposed to null.

What are the testing steps for this pull request?

Register a bootc host with a /etc/rhsm/facts/bootc.facts file that looks like what's above.