fix(select): make renderValue and placeholder work together (#413)

Author: mattrothenberg

.changeset/fix-select-rendernull.md

@@ -0,0 +1,19 @@
+---
+"@cloudflare/kumo": patch
+---
+
+fix(Select): TypeScript inference with `strictNullChecks` and `renderValue`/`placeholder` interaction
+
+**TypeScript fix:** Under `strictNullChecks`, using `value={objectOrNull}` would cause `T` to be inferred as `never`, making callbacks like `onValueChange` and `renderValue` unusable. This is now fixed.
+
+**Runtime fix:** `renderValue` is now only called with non-null values. When value is null, the `placeholder` is shown instead. Previously, `renderValue` would receive `null` at runtime despite being typed as `(value: T) => ReactNode`.
+
+```tsx
+// Recommended pattern
+<Select
+  placeholder="Select..."
+  value={value}
+  onValueChange={setValue} // value is T | null (works with strictNullChecks)
+  renderValue={(v) => v.name} // v is T (non-null), no defensive coding needed
+/>
+```