Fix model runs with no modifications.
Fix model runs with no modifications.

* Model runs with no modifications that previously had modifications
were throwing exceptions, but this case should be supported. If there
are pieces but no censuses, we should throw an exception because the
model results will be incorrect. However, if there are no pieces
but a census, we can just delete the census and run the model.

Refs #921

lliss/model-my-watershed,lliss/model-my-watershed,project-icp/bee-pollinator-app,kdeloach/model-my-watershed,lliss/model-my-watershed,project-icp/bee-pollinator-app,kdeloach/model-my-watershed,project-icp/bee-pollinator-app,project-icp/bee-pollinator-app,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,kdeloach/model-my-watershed,kdeloach/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,lliss/model-my-watershed,WikiWatershed/model-my-watershed,lliss/model-my-watershed,kdeloach/model-my-watershed