fix truthiness check; safety check around bin count

[mike-kaimika]

This PR corrects a few minor bugs that were not directly found in the wild, but could be an issue

• Fix exception through if the length of a dataset (which would be the number of bins) is called on a new dataset that does not have a backing data from the database (e.g., unsaved)
• Fixes an issue with conditional checks against a dataset. E.g., True if dataset else False. Because len is defined, that is used as truthiness rather than the default on an ORM model, which is to just return True (whether the model is saved or not). The included bool method takes precedence and both corrects the logic, and also prevents accidental usage of len in a conditional statement that would make a database call that might not be expected

[joefutrelle]

LGTM, good catch