Fix nullable: false validation

[leNEKO]

Context

(See this issue #227)

For some reason nullable is implicitly true (see https://github.com/thephpleague/openapi-psr7-validator/pull/155/files#r1587568801)

But trying to validate an explicitly"nullable": false property will crash with a TypeError instead of throwing an InvalidBody Exception.

Modification

• checking if the $this->parentSchema->type is a proper array instead of checking if it is not a string
• adding a non-regression test