Fix == / != to match JavaScript abstract equality for null

[erikgaal]

Problem

JsonLogic's value proposition is that a rule evaluates identically on the front end (json-logic-js) and the back end (this library). For null, they diverge:

// json-logic-js
jsonLogic.apply({"==": [null, 0]})  // => false

// json-logic-php
JWadhams\JsonLogic::apply(['==' => [null, 0]]);  // => true  (wrong)

In JavaScript, null is loosely-equal only to null/undefined, never to 0, false, or "". PHP's native == coerces null to those, so == / != return the opposite of the reference implementation whenever an operand is null. This bites real rules: a missing or unanswered field compared with {"==": [{"var": "x"}, 0]} is false in the browser but true here, so client-side visibility and server-side validation disagree.

Fix

Guard the null case in == and != so null is loosely-equal only to null, matching json-logic-js. Non-null comparisons are untouched (PHP 8 already matches JS for those).

'==' => function ($a, $b) {
    if ($a === null || $b === null) {
        return $a === null && $b === null;
    }
    return $a == $b;
},

Tests

The shared jsonlogic.com tests.json parity suite still passes (it has no null-equality cases, which is how this gap went unnoticed). Added tests/NullLooseEqualityTest.php covering null against null, 0, false, "", "0", [] for both == and !=, plus non-null sanity cases.

Scope is intentionally limited to the null divergence; broader JS-coercion edge cases are left as-is.