fix(api): return manifest pull stats for digest-only pulls (PROJQUAY-9877)
When images are pulled by digest only (not by tag), the API endpoint
was returning 0 for manifest_pull_count because it ignored manifest_stats
when tag_stats was None.
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Co-authored-by: Brady Pratt <bpratt@redhat.com>
Co-authored-by: Claude <noreply@anthropic.com>