$schema->nullable can be null as of cebe/php-openapi 1.6

[canvural]

Hello,

I just updated to cebe/php-openapi 1.6.0 and got an error in my test suite.

As of 1.6.0, $schema->nullable can be null : https://github.com/cebe/php-openapi/blob/master/src/spec/Schema.php#L130

This causes error here: https://github.com/thephpleague/openapi-psr7-validator/blob/master/src/Schema/SchemaValidator.php#L60 Argument #2 ($nullable) must be of type bool, null given

Currently I don't have time send a PR with a fix. So just reporting it.

[cebe]

Linking related discussions for reference:

• Validation issue when using allOf with $ref #118
• nullable oneOf/anyOf value rejected #146

[scaytrase]

Implemented quick fix