Generate the null value

Property definitions

nitc $ AbstractCompilerVisitor :: null_instance
	# Generate the `null` value
	fun null_instance: RuntimeVariable
	do
		var t = compiler.mainmodule.model.null_type
		var res = new RuntimeVariable("((val*)NULL)", t, t)
		return res
	end
src/compiler/abstract_compiler.nit:1792,2--1798,4