handle special null case when hydrating

src/runtime/internal/dom.ts

@@ -162,7 +162,7 @@ export function claim_text(nodes, data) {
 	for (let i = 0; i < nodes.length; i += 1) {
 		const node = nodes[i];
 		if (node.nodeType === 3) {
-			node.data = data;
+			node.data = '' + data;
 			return nodes.splice(i, 1)[0];
 		}
 	}

test/hydration/samples/dynamic-text-nil/_after.html

@@ -1,2 +1,2 @@
-<p></p>
+<p>null</p>
 <p>undefined</p>