Can value be null? (according to current knowledge)

Property definitions

nitc $ AbstractCompilerVisitor :: maybe_null
	# Can value be null? (according to current knowledge)
	fun maybe_null(value: RuntimeVariable): Bool
	do
		return value.mcasttype isa MNullableType or value.mcasttype isa MNullType
	end
src/compiler/abstract_compiler.nit:1544,2--1548,4