fix: handle null sharding key in COPY parser

[levkk]

In case COPY data has a \N (configurable) null value for the sharding key, the row is sent to all shards.

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/parser/copy.rs	96.87%	1 Missing ⚠️

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