Handle nullable DirectiveNode#astNode in SchemaValidationContext

[spawnia]

TypeError : Return value of GraphQL\Type\SchemaValidationContext::GraphQL\Type\{closure}() must be an instance of GraphQL\Language\AST\DirectiveDefinitionNode, null returned
 /workdir/vendor/webonyx/graphql-php/src/Type/SchemaValidationContext.php:227
 /workdir/vendor/webonyx/graphql-php/src/Utils/Utils.php:172
 /workdir/vendor/webonyx/graphql-php/src/Type/SchemaValidationContext.php:228
 /workdir/vendor/webonyx/graphql-php/src/Type/SchemaValidationContext.php:154
 /workdir/vendor/webonyx/graphql-php/src/Type/Schema.php:527
 /workdir/vendor/webonyx/graphql-php/src/Type/Schema.php:480
 /workdir/src/Console/ValidateSchemaCommand.php:43

[coveralls]

Coverage decreased (-0.006%) to 86.241% when pulling fe13957 on spawnia:directive-ast-node-nullable into c1e9c5e on webonyx:master.

[vladar]

LGTM 👍