Add cxjs_nullif()

[Lightning11wins]

This PR will add the cxjs_nullif() on the client, which runs with the same logic as the server-side version.

[greptile-apps]

Greptile Summary

This PR adds client-side support for cxjs_nullif() in the rendering helpers. The main change is:

• A new two-argument helper using coercive equality to mirror server-side comparison.

Confidence Score: 4/5

The null second-operand path needs a fix before merging.

• cxjs_nullif(nonNullValue, null) returns null instead of preserving the first value.
• This can make client and server expression results differ.

centrallix-os/sys/js/ht_render.js

Important Files Changed

Filename	Overview
centrallix-os/sys/js/ht_render.js	Adds cxjs_nullif(), but a null second operand incorrectly nullifies a non-null first operand.

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
centrallix-os/sys/js/ht_render.js:324-325
**Null Second Operand Drops Value**

When `a` is non-null and `b` is null, this branch returns null instead of `a`. For example, `cxjs_nullif('value', null)` produces null, so client expressions can disagree with the server-side `NULLIF` result.

```suggestion
    if (a == null || a == b) return null;
    return a;
```

_{Reviews (1): Last reviewed commit: "Add cxjs_nullif()." | Re-trigger Greptile}

[Lightning11wins]

This PR is cleared for human review.