Don't crash Form's change handler outside a form

Form reads the closest form on every change. A field rendered through a
portal (a Modal) has no form ancestor in the DOM, so FormData(null) threw.
Change handlers now get an empty object instead.

src/form/Form.tsx

@@ -37,9 +37,12 @@ export default function Form<
             onChange={(e) => {
                 e.preventDefault();
                 const taregtEl = e.target as HTMLElement;
-                const formEl = taregtEl.closest("form") as HTMLFormElement;
-                const formData = new FormData(formEl);
-                const data = Object.fromEntries(formData.entries()) as T;
+                const formEl = taregtEl.closest("form") as HTMLFormElement | null;
+                const data = formEl
+                    ? (Object.fromEntries(
+                          new FormData(formEl).entries(),
+                      ) as T)
+                    : ({} as T);
                 props.changeHandler?.(e, data);
                 props.onChange?.(e);
             }}