Fix MySQL query to compute number of rows per table (#1128)

The CAST should target `unsigned` rather than `integer`, so that we are compatible with MySQL 5.7. Also empty tables might have NULL entries, which we transform to zero entries here, as expected by the Lisp code.

Fixes #1127.

src/sources/mysql/sql/list-table-rows.sql

@@ -4,7 +4,7 @@
 --         excluding
 --         filter-list-to-where-clause excluding
     SELECT table_name,
-           cast(data_length/avg_row_length as integer)
+           coalesce(cast(data_length/avg_row_length as unsigned), 0)
       FROM information_schema.tables
     WHERE     table_schema = '~a'
           and table_type = 'BASE TABLE'