Rename function isEmpty to checkIfEmpty

[jlouche]

I just rename the function isEmpty to checkIfEmpty.

If validate.js is not used directly in a project but via a library (which it uses validate.js), the following error occurs:

UnCaught TypeError: unable to assign read-only property 'isEmpty' of function '(attributes, constraints, options) { options = v.extend ({}, v.options, options);

The problem seems to be that the isEmpty function is already defined by default as a prototype.