fix: ignore insecure-only imager assets

[kastakhov]

Pull Request

What? (description)

Fix imager asset presence checks so forceInsecure alone does not make a container asset configured.

Changes:

• add ContainerAsset.IsSet() based on source fields only
• default baseInstaller when only forceInsecure is set
• skip image cache extraction when no image cache source is set
• add tests for insecure-only assets and base installer defaulting

Why? (reasoning)

imager --insecure can set imageCache.forceInsecure: true without an image cache image. That made imager try to pull an empty image ref.

Closes #13824

Acceptance

Please use the following checklist:

• you linked an issue (if applicable)
• you included tests (if applicable)
• you ran conformance (make conformance)
• you formatted your code (make fmt)
• you linted your code (make lint)
• you generated documentation (make docs)
• you ran unit-tests (make unit-tests)

Targeted tests run:

GOCACHE=/tmp/talos-go-cache go test ./pkg/imager/profile ./pkg/imager