Fix @defer on lazy objects when Dataloader isn't used

[rmosolgo]

Fixes #5490

.run_isolated(&block) may receive a block which already references a certain dataloader instance, and might call dataloader.lazy_at_depth(...) on it. In that case, the lazy is registered with the first NullDataloader instance, not the newly-created one. Then when .run is called on the newly-created instance, it doesn't find any of those registered lazies.

By reusing self instead of making a new instance, the references inside the given block are still "correct" -- the lazies given to dataloader.lazy_at_depth are properly found and resolved when self.run is called later on.