nullable rule doesn't work in conjunction with anyof* rules

[dkbarn]

Used Cerberus version: 1.3.4

Use-case abstract

There is a value in my schema that can be an integer or a string or null.

Bug report

This seems like a bug, because as soon as I try to introduce an anyof* rule, the nullable rule is no longer respected.

nullable works with only a single type. The following validate() call returns True.

schema = {"foo": {"type": "integer", "nullable": True}}
Validator(schema).validate({"foo": None})

nullable stops working with multiple possible types. All of the following validate() calls erroneously return False.

schema = {"foo": {"anyof_type": ["integer", "string"], "nullable": True}}
Validator(schema).validate({"foo": None})

schema = {"foo": {"anyof": [{"type": "integer"}, {"type": "string"}, {"nullable": True}]}}
Validator(schema).validate({"foo": None})

schema = {"foo": {"anyof": [{"type": "integer", "nullable": True}, {"type": "string", "nullable": True}, {"nullable": True}]}}
Validator(schema).validate({"foo": None})