fix(jambo): Add NullTypeParser implementation and tests (#2)

jambo/parser/__init__.py

@@ -7,6 +7,7 @@ from .const_type_parser import ConstTypeParser
 from .enum_type_parser import EnumTypeParser
 from .float_type_parser import FloatTypeParser
 from .int_type_parser import IntTypeParser
+from .null_type_parser import NullTypeParser
 from .object_type_parser import ObjectTypeParser
 from .ref_type_parser import RefTypeParser
 from .string_type_parser import StringTypeParser
@@ -22,6 +23,7 @@ __all__ = [
     "BooleanTypeParser",
     "FloatTypeParser",
     "IntTypeParser",
+    "NullTypeParser",
     "ObjectTypeParser",
     "StringTypeParser",
     "RefTypeParser",