Handle 'null' with defaults

[whophi]

This uses a default value in each of the three value functions, jimp_bool, jimp_number, and jimp_string, that are used if 'null' is encountered.

If 'null' is encountered instead of an object, the object is left unchanged. This means the object to be filled in is left as is and can therefore be used as the default.

If 'null' is encountered instead of an array, the provided array is left unchanged. As before, this means the provided array can be seen as the default.