fix(runtime): do not render empty Fragment (#2667)

This commit fixes the bug when returning an empty Fragment which makes
it undefined. This is because unescapeHTML (raw) will create a String object
with an 'undefined' string.

To fix this we need to check if the `children` is null or undefined
before marking a string as raw.

packages/astro/src/runtime/server/index.ts

@@ -131,6 +131,9 @@ export async function renderComponent(result: SSRResult, displayName: string, Co
 	const children = await renderSlot(result, slots?.default);
 
 	if (Component === Fragment) {
+		if (children == null) {
+			return children;
+		}
 		return unescapeHTML(children);
 	}