Add tests for DB_Table.get_row and fix a bug

[radeusgd]

Currently we seem to have little to no tests for Table.get_row (and DB_Table counterpart).

As noticed in a review, we actually have a bug in the DB_Table implementation - calling get_row -1 on an empty table will currently raise Index_Out_Of_Bounds error while the expected behaviour is to return if_missing value (Nothing) by default. This went unnoticed because we are lacking tests.

• Add tests for get_row both on Table and DB_Table (may need a sort before to ensure consistent ordering in DB).
  • Specifically add tests for edge cases like empty table, especially get_row -1 but also perhaps get_row -1 and get_row -2 on row table.