Unexpected validation error with nullable allOf

How to reproduce:

```python
from openapi_schema_validator import validate, OAS30Validator, OAS31Validator

schema = {
  "allOf": [
    {"type": "string"},
  ],
  "nullable": True,
}

validate(None, schema, cls=OAS30Validator)
# --> None for not nullable

validate(None, schema, cls=0AS31Validator)
# --> None is not of type 'string'
```

Seems related to #20