Definition of isEmpty

[phorward]

Hello, regarding the failing tests: This occurs when something different then a str is given to isEmpty.
A general question on this: Why can't isEmpty/isValid etc. just rely that the provided value is of the given type of the bone, which is parsed using fromClient? Either the tests must be changed, or the generic concept of VIUR. Well okay, I think this is a generall discussion topic, just fixing it now this way and see the rest later on.

Originally posted by @phorward in #514 (comment)