fix: check for falsy values in spread (#11388)

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

packages/svelte/src/internal/client/reactivity/props.js

@@ -169,7 +169,7 @@ const spread_props_handler = {
 	has(target, key) {
 		for (let p of target.props) {
 			if (is_function(p)) p = p();
-			if (key in p) return true;
+			if (p != null && key in p) return true;
 		}
 
 		return false;

packages/svelte/tests/runtime-legacy/samples/prop-p-is-null/Child.svelte

@@ -0,0 +1,6 @@
+<script>
+	$: x = Object.keys($$restProps).length;
+	$: y = Object.keys($$props).length;
+</script>
+
+{x} {y}

packages/svelte/tests/runtime-legacy/samples/prop-p-is-null/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: '0 0'
+});

packages/svelte/tests/runtime-legacy/samples/prop-p-is-null/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	import Child from './Child.svelte'
+</script>
+
+<Child {...undefined} />