chore: harden addon warning logging and empty drain

[atharvaHJoshi]

What this PR does

Two small hardening fixes found during issue investigations:

1. pkg/actions/addon/create.go — logger.Warning used the %w verb (intended for
   fmt.Errorf/error wrapping). Loggers that follow the go-kit convention render
   %w as a literal %!w(...) placeholder, so the actual error text was garbled.
   Switched to %v.

2. pkg/actions/nodegroup/drain.go — Drainer.Drain now returns early when it is
   asked to drain zero nodegroups, instead of logging "starting parallel draining"
   and clearing with nothing to do.

Validation

• go build ./...
• golangci-lint run --timeout=30m ./pkg/...
• go test ./pkg/actions/...

Pre-existing, unrelated failures in pkg/info and pkg/iam/oidc are environmental
(kubectl/cfssl missing locally) and remain unchanged.

[github-actions]

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