Assert partitions are empty before ALTERs

[jcjones]

Rather than simply inferring that a partition is empty, we can check it concretely through a simple SELECT statement, like:

SELECT * FROM table PARTITION(p_20230724) LIMIT 1;

If any data comes back, then we have failed an invariant.

[jcjones]

This is a little tricky to get the tests right for, as it considerably increases the number of database queries in the common case. Some of this is going on in branch 62-assert-empty.