Handle when an entry in the definitions section is not an object

[SDParish]

i.e. it is a primitive type and has no properties property

As far as I can see primitive types are allowed as definitions entries, however the code in GenerateModels.php::generate() creates a new class for each, and assumes that the properties property both exists and is an array, leading to an error when it tries to use it and it doesn't exist/is treated as null.

The code should check the type specified, and act accordingly.
Whether that means no class generated (and so the primitive type would be used elsewhere rather than a Model), or a wrapper Model that holds the primitive (and applies pattern/formats/other properties as per the spec) can be decided when implementing.