Codegen doesn't support nullable types

失败发生在 rpdk/java/pojo_resolver.py 的 _java_property_type 中，该函数由 cfn-cli generate 调用；从这里开始，并使用 nullable Canceled schema 重现该问题。当 ["integer", "null"] 属性能够在没有 unhashable-list 错误的情况下成功生成时，即表示完成。

This is the property that I’m trying to define:

    "Canceled": {
        "description": "POSIX Timestamp of cancellation of this downtime (null if not canceled)",
        "type": ["integer", "null"]
    },

This should be ok according to JsonSchema, but cfn-cli generate fails and rpdk.log contains this:

Traceback (most recent call last):
  File "/Users/.../programming/aws-cloudformation-rpdk-nightly/env/lib/python3.7/site-packages/rpdk/core/cli.py", line 95, in main
    args.command(args)
  File "/Users/.../programming/aws-cloudformation-rpdk-nightly/env/lib/python3.7/site-packages/rpdk/core/generate.py", line 15, in generate
    project.generate()
  File "/Users/.../programming/aws-cloudformation-rpdk-nightly/env/lib/python3.7/site-packages/rpdk/core/project.py", line 190, in generate
    return self._plugin.generate(self)
  File "/Users/.../programming/aws-cloudformation-rpdk-nightly/env/lib/python3.7/site-packages/rpdk/java/codegen.py", line 255, in generate
    pojos = pojo_resolver.resolve_pojos()
  File "/Users/.../programming/aws-cloudformation-rpdk-nightly/env/lib/python3.7/site-packages/rpdk/java/pojo_resolver.py", line 58, in resolve_pojos
    java_property_map[prop_name] = self._java_property_type(prop_schema)
  File "/Users/.../programming/aws-cloudformation-rpdk-nightly/env/lib/python3.7/site-packages/rpdk/java/pojo_resolver.py", line 88, in _java_property_type
    return primitive_types_map[json_type]
TypeError: unhashable type: 'list'