Deprecation notice on PHP 8.1

[morozov]

Constraint#incrementPath() calls strlen() on NULL which leads to a notice:

$ php --version
PHP 8.1.0-dev (cli) (built: Feb 20 2021 21:48:33) ( NTS )

$ composer --version
Deprecation Notice: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in vendor/justinrainbow/json-schema/src/JsonSchema/Constraints/Constraint.php:48
Composer version 2.0.999-dev+source @release_date@

Execution flow:

1. https://github.com/justinrainbow/json-schema/blob/f4f0c3457811006f861c466a10c6c24f3e92f9ae/src/JsonSchema/Constraints/UndefinedConstraint.php#L34-L40
2. https://github.com/justinrainbow/json-schema/blob/f4f0c3457811006f861c466a10c6c24f3e92f9ae/src/JsonSchema/Constraints/Constraint.php#L43-L49