fix _reset_eval_dataloader() for IterableDataset (#1560)

* removed if dl from _reset_eval_dataloader()

* changed to if dl != None to be more safe

* hints from pep8speaks

Co-authored-by: ybrovman <ybrovman@ebay.com>

pytorch_lightning/trainer/data_loading.py

@@ -219,7 +219,7 @@ class TrainerDataLoadingMixin(ABC):
                     ' this off for validation and test dataloaders.')
 
         # add samplers
-        dataloaders = [self.auto_add_sampler(dl, train=False) for dl in dataloaders if dl]
+        dataloaders = [self.auto_add_sampler(dl, train=False) for dl in dataloaders if dl is not None]
 
         num_batches = 0