cli/command/manifest: allow unsetting variant and os-version in annotate

[GitNimay]

• Fixes: docker annotate can't remove variant #4352

- What I did

docker manifest annotate only applied --variant and --os-version when the value was non-empty, so there was no way to remove a variant or OS version from a manifest entry. These flags are now applied whenever they are passed, so an empty value unsets the field. Not passing the flag keeps the existing value, as before.

Before:

$ docker manifest annotate myrepo/list myrepo/image --variant ""
# variant is unchanged (e.g. still "v8")

After:

$ docker manifest annotate myrepo/list myrepo/image --variant ""
# variant is removed

- How I did it

Record whether --variant and --os-version were set (cmd.Flags().Changed), and use that instead of a non-empty check.

- How to verify it

go test ./cli/command/manifest/ -run TestManifestAnnotate

The new TestManifestAnnotateUnsetFields test fails without this change.

- Human readable description for the release notes

Fix `docker manifest annotate` ignoring an empty `--variant` or `--os-version`, which prevented unsetting these fields.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[GitNimay]

@thaJeztah @vvoland PTAL when you have a moment.

The host (macos-14) failure looks unrelated: TestRunBuildFromGitHubSpecialCase couldn't git clone from GitHub on the runner (network). Could you re-run that job? check-changelog and validate-milestone just need an impact/ label and a milestone. Thanks!