fix: handle nil auth when pulling unauthenticated images

[porridge]

Summary

• Fix nil pointer panic in internal/main.go:81 when credential providers are configured but no credentials match the image being pulled (e.g. public images).
• getAuthsForImage intentionally returns a nil AuthConfig entry for unauthenticated pulls, but the logger call in Run() dereferenced it to extract ServerAddress and Username.
• Extract pullLogger helper that safely builds a logger for pull attempts, omitting auth fields when auth is nil.

Test plan

• Observed the panic on an OCP cluster when prefetching docker.io/library/busybox with credential provider integration enabled
• go build ./internal/ compiles cleanly

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