Support null and undefined in JrxNode type

Split JrxNode into JrxElement (concrete branded object) and JrxNode
(JrxElement | null | undefined), mirroring React's ReactElement vs
ReactNode pattern. Components can now return null/undefined to render
nothing.

- Rename isJrxNode to isJrxElement (deprecated alias kept)
- Handle null-returning components in jsx/jsxs factories
- Add tests for null/undefined returns, children filtering, render()
  rejection

Co-authored-by: Ona <no-reply@ona.com>

src/types.ts

@@ -19,12 +19,14 @@ export const JRX_NODE = Symbol.for("jrx.node");
 export const FRAGMENT = Symbol.for("jrx.fragment");
 
 /**
- * A node in the intermediate JSX tree.
+ * A concrete element in the intermediate JSX tree.
  *
  * Created by the `jsx` / `jsxs` factory functions and consumed by `render()`
  * which flattens the tree into a json-render `Spec`.
+ *
+ * Analogous to React's `ReactElement`.
  */
-export interface JrxNode {
+export interface JrxElement {
   /** Brand symbol — always `JRX_NODE` */
   $$typeof: typeof JRX_NODE;
 
@@ -38,7 +40,7 @@ export interface JrxNode {
   props: Record<string, unknown>;
 
   /** Child nodes */
-  children: JrxNode[];
+  children: JrxElement[];
 
   // -- Reserved / meta fields (extracted from JSX props) --
 
@@ -58,6 +60,14 @@ export interface JrxNode {
   watch: Record<string, ActionBinding | ActionBinding[]> | undefined;
 }
 
+/**
+ * Any value that can appear as a JSX child or component return value.
+ *
+ * Analogous to React's `ReactNode` — includes `null` and `undefined`
+ * so components can conditionally render nothing.
+ */
+export type JrxNode = JrxElement | null | undefined;
+
 // ---------------------------------------------------------------------------
 // JrxComponent — a function usable as a JSX tag that maps to a type string
 // ---------------------------------------------------------------------------
@@ -65,7 +75,7 @@ export interface JrxNode {
 /**
  * A jrx component function. Works like a React function component:
  * when used as a JSX tag (`<Card />`), the factory calls the function
- * with props and gets back a JrxNode.
+ * with props and gets back a JrxNode (which may be null/undefined).
  */
 export type JrxComponent = (props: Record<string, unknown>) => JrxNode;
 
@@ -85,7 +95,7 @@ export type JrxComponent = (props: Record<string, unknown>) => JrxNode;
 export function component(typeName: string): JrxComponent {
   // Import createNodeFromString lazily to avoid circular dep
   // (jsx-runtime imports types). Instead, we build the node inline.
-  return (props: Record<string, unknown>) => {
+  return (props: Record<string, unknown>): JrxElement => {
     return {
       $$typeof: JRX_NODE,
       type: typeName,
@@ -110,21 +120,21 @@ function filterReserved(props: Record<string, unknown>): Record<string, unknown>
   return out;
 }
 
-function normalizeChildrenRaw(raw: unknown): JrxNode[] {
+function normalizeChildrenRaw(raw: unknown): JrxElement[] {
   if (raw == null || typeof raw === "boolean") return [];
   if (Array.isArray(raw)) {
-    const result: JrxNode[] = [];
+    const result: JrxElement[] = [];
     for (const child of raw) {
       if (child == null || typeof child === "boolean") continue;
       if (Array.isArray(child)) {
         result.push(...normalizeChildrenRaw(child));
       } else {
-        result.push(child as JrxNode);
+        result.push(child as JrxElement);
       }
     }
     return result;
   }
-  return [raw as JrxNode];
+  return [raw as JrxElement];
 }
 
 // ---------------------------------------------------------------------------
@@ -140,10 +150,13 @@ export interface RenderOptions {
 // Type guard
 // ---------------------------------------------------------------------------
 
-export function isJrxNode(value: unknown): value is JrxNode {
+export function isJrxElement(value: unknown): value is JrxElement {
   return (
     typeof value === "object" &&
     value !== null &&
-    (value as JrxNode).$$typeof === JRX_NODE
+    (value as JrxElement).$$typeof === JRX_NODE
   );
 }
+
+/** @deprecated Use `isJrxElement` instead. */
+export const isJrxNode = isJrxElement;