Bug report use type `nullable` and `anyOf` in express-openapi-validator

gistfile1.txt

**Describe the bug**

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Hiya. Thanks for putting together this awesome library.

We're trying to validate a response schema as follows:

```
          properties:
            name:
              type: string
            value:
              # type: string # <--- validator needs this here to use `nullable`, however it still ignores it
              nullable: true
              anyOf:
                - type: string
                - type: boolean
                - type: integer
                - type: object
                - type: array
```

**To Reproduce**

Steps to reproduce the behavior.

Use the openAPI schema above, validator with `validateResponses: true` and return a response as follows: 

```
{
   "name": "some key",
   "value": null
}
```

**Actual behavior** 

```
Error: /response/value must be string, /response/value must be boolean, /response/value must be integer, /response/value must be object, /response/value must be array, /response/value must match a schema in anyOf
```

A clear and concise description of what happens.

**Expected behavior**
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**Examples and context**
An example or relevant context e.g. an OpenAPI snippet, an Express handler function snippet