Fix: Distinguish missing vs null in var resolution (use default only when missing)

[andras-markos]

Supersedes #58; this PR contains only the fix.

This fixes var resolution so a default is applied only when the path is missing, not when the value is explicitly null.

Why

Per the JsonLogic docs, the second var argument is a default “for values that might be missing.” Treating null as missing deviates from that intent and leads to surprising results.

What changed

• Introduced an internal MISSING sentinel to track “not found” during path traversal.
• Map access now uses containsKey to differentiate an absent key from a present key with null.
• Array access returns MISSING for out-of-bounds indices.
• evaluate(JsonLogicVariable, …) applies the default only when the traversal yields MISSING; explicit null is returned as null.

Examples

• {"var": ["user.age", 42]} with {"user":{"age": null}} → null (no default)
• {"var": ["items.2", "missing"]} with {"items":[10,20]} → "missing"
• {"var": ""} returns the entire data object (same instance)

Tests

Added coverage to VariableTests:

• Missing nested key falls back to default.
• Present leaf null returns null (no default).
• Intermediate null or non-traversable types return null.
• Array index in-bounds returns the element; out-of-bounds uses default.
• Top-level numeric index over lists works.
• Empty var key returns the original data object.

Compatibility

Behavior change for callers who relied on “default-for-null.” This aligns with the JsonLogic spec and other interpreters.