In a developer org, although validation rule lists are empty, checks are performed against data. E.g. trying to create an Entitlement record may lead to an error "The number of Remaining Work Orders must be less than or equal to the number of Work Orders per Entitlement" or "The end date must be equal to or later than the start date". That makes sense of course, but where can I find a list of all these data checks?
Salesforce has a lot of standard validation that goes on. Accordingly, there's not one single page describes every possible validation. It'd be a very, very large list. Instead, there's the Standard Objects page. Validations and special access rules are mostly listed on the relevant object. Additional places to check are the in the Help & Training for a given object. Ultimately, you'll have to do some research when you start working with a new object.