Codegen doesn't support nullable types

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'
```