Invalid foreach in mongodb_field_storage.module, line 157

It looks like, occasionally (not sure exactly how), $entity->$field_name isset and is iteratable, but doesn't actually contain anything. I *suspect* that this has to do with json_deocde, but I'm unsure. This results in us getting an "invalid foreach" error on $values[0] on line 157. It's a fairly easy fix, just need to change isset to !empty