`server_signature_keys` has all nullable columns, `server_name` and `key_id` are especially dodgy

This issue has been migrated from [#16646](https://github.com/matrix-org/synapse/issues/16646).

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Start by locating the schema or migration that defines the server_signature_keys table, then inspect how server_name and key_id are written and queried. The issue provides no expected nullability rules or acceptance criteria, so completion requires establishing the intended constraints before changing the schema and its related tests.

Written by the indexing model from the issue text.