Don't generate empty argument objects

[jorrit]

I have a fairly large API and as such a lot of PHP files are generated by the OQM code generator.

Most of the field in my schema have no arguments, but right now the OQM generates an empty ArgumentsObject class for each object field.

This PR enhances the code generator so an ArgumentsObject is only generated when a field accepts arguments.

[jorrit]

@mghoneimy I've tried this change on my code base and it removes a large amount of useless classes.