In SQL, return an empty array when a schema has no tables

[seancolsen]

Before

• msar.get_table_info would return NULL for a schema with no tables.
• This bubbled up to the API layer, meaning that tables.list would sometimes return null.
• This bubbled up to the front end creating this bug.

After

• msar.get_table_info returns an empty array for a schema with no tables.
• The return type for tables.list can remain as-documented list[TableInfo] (instead of Optional[list[TableInfo]]), and the return type in the front end can also remain as currently typed. The typing is simpler.
• Bug fixed.

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• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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