Delete `map` keys

Either `#update()` needs to support deleting map keys (maybe triggered by `null` values?), or `#remove()` semantics need to be updated to support deleting map keys.

Since we already have validation that `#update()` cannot set map keys to `null`, I propose that `null` values in here will generate a `DELETE [""] FROM WHERE ` statement.

Right now, to "delete" a map key, we are required to set it to some sort of empty value. This is fine for most maps, but causes problems with a `map`.