pr-upload: fix --warn-on-upload-failure functionality

For bottle uploads that would be skipped due to preupload_check warning about
skipped uploads and returning early, the old code errored out due to JSON schema
validation failure since the upload was allowed to resume:

    "error"=>"value at /manifests/0/annotations/org.opencontainers.image.ref.name is not a string"

This change ensures that the upload is effectively skipped if preupload_check
returns no result.

Library/Homebrew/github_packages.rb

@@ -260,6 +260,8 @@ class GitHubPackages
     # We run the preupload check twice to prevent TOCTOU bugs.
     result = preupload_check(user, token, skopeo, formula_full_name, bottle_hash,
                              keep_old:, dry_run:, warn_on_error:)
+    # Skip upload if preupload check returned early.
+    return if result.nil?
 
     formula_name, org, repo, version, rebuild, version_rebuild, image_name, image_uri, keep_old = *result