Allow null for client_secret
Allow null for client_secret

Changed `client_secret` declaration in schema to allow NULL values
MattermarkStudios/oauth2-server-php,svycka/oauth2-server-php,Peercraft/oauth2-server-php,sariabod/oauth2-server-php,eric-admin/oauth2-server-php,kazitanvirahsan/oauth2-server-php,bshaffer/oauth2-server-php,wanadev/oauth2-server-php,SixSrt/oauth2-server-php,tobias74/oauth2-server-php,mondor/oauth2-server-php,Wilt/oauth2-server-php