Add NOT NULL for sharding key on issue_email_participants

What does this MR do and why?

Add NOT NULL for shardAdd NOT NULL for sharding Adds NOT NULL constraint on sharding key column namespace_id on table: issue_email_participants.

This merge request is the last step towards transforming the desired sharding key into a sharding key.

This involves the following changes:

• Adding a not null constraint for the sharding key column (namespace_id).
• Setting namespace_id as sharding_key of issue_email_participants.

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