Handle case where no diagnosis/assessment exist in the graph

[alyssadai]

Currently, the query tool emits some error (warning?) messages about "no diagnosis options" being fetched (and similarly for assessment) if the data in the graph doesn't contain any assessment/diagnosis data.

This however doesn't actually prevent you from querying the data that is in the graph (e.g., with an empty query). We should probably either remove this error message or make it more informative to indicate that it doesn't necessarily mean there is a problem with the database.