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

[andras-markos]

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.

[jamsesso]

Please remove

[jamsesso]

I'm happy with the code fixes but I'm not onboard with the CI changes. Those should be a separate PR. Because the two are mixed in here, I'll have to decline.

[jamsesso]

Remove this change

[jamsesso]

Remove this change

[jamsesso]

I'm fine upgrading Gradle, but can this be done in another PR?

[jamsesso]

Same comment as before - bundling two different changes in a PR (missing + gradle changes) isn't ideal. I don't want to discuss two changes in a single PR. the code for MISSING seems fine and can be deployed, but this is something I'd like to think about more.

[andras-markos]

Sorry about that, that was not my intention.
I have opened a new PR for just the fix.
I only wanted to add the rest of the changes to the sailthru fork.
This PR can be closed.