- null foreign key on a m2o doesn't trigger a lazyload [ticket:803]

- null foreign key on a m2o doesn't trigger a lazyload [ticket:803]
- slight simpliication to mapper.populate_instance()
- lamenting the different codepaths between query._get() and DeferredLoader.lazyload()
- query._get() uses all()[0] for single-row load to avoid complexity of first() (same as LazyLoader)