updateReturning surprising behavior when assignments are empty

When using `updateReturning` as an optimized of `update` plus `select`, with a variable set of assignments, it's surprising that no rows are returned when the assignments are empty.

The behavior does make sense for use cases where you're only interested in the changes, so it seems like beam should have two functions. Perhaps `updateReturningChanged` and `updateReturningMatched`. The latter would perform a `SELECT` when the assignments are empty, because `UPDATE table SET WHERE ...` isn't valid syntax.