A null value can be matched.

Hi there,

When testing my JSON OAuth responses, I tried to compare with the following pattern;
```json
{
    "access_token": @string@,
    "expires_in": 3600,
    "token_type": "bearer",
    "scope": null,
    "refresh_token": @string@
}
```

Which gave me the error `There is no element under path [scope] in pattern.`.

To fix this, I came up with a NullMatcher so that the following pattern can be used;
```json
{
    "access_token": @string@,
    "expires_in": 3600,
    "token_type": "bearer",
    "scope": @null@,
    "refresh_token": @string@
}
```