Exclude placeholder producer client IDs from duplicate summaries

## Problem Producer connection summaries already treat the literal null client ID as missing metadata, but duplicate detection counts repeated null placeholders as a real duplicate client. The same rows therefore produce contradictory missing and duplicate diagnostics. ## Expected behavior Use one normalized identity rule for completeness, unique counts, and duplicate detection. Case-insensitive null placeholders remain missing and never count as real clients. ## Verification Add a focused summary test with null and padded uppercase NULL values.