allow slots to have missing props in SSR - fixes #3322

src/compiler/compile/render_ssr/handlers/Slot.ts

@@ -8,7 +8,7 @@ export default function(node: Slot, renderer: Renderer, options: RenderOptions)
 
 	const slot_data = get_slot_data(node.values, true);
 
-	const arg = slot_data.length > 0 ? `{ ${slot_data.join(', ')} }` : '';
+	const arg = slot_data.length > 0 ? `{ ${slot_data.join(', ')} }` : '{}';
 
 	renderer.append(`\${$$slots${prop} ? $$slots${prop}(${arg}) : \``);
 

test/runtime/samples/component-slot-let-missing-prop/Bar.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let thing;
+</script>
+
+<p>{thing}</p>

test/runtime/samples/component-slot-let-missing-prop/Foo.svelte

@@ -0,0 +1 @@
+<slot></slot>

test/runtime/samples/component-slot-let-missing-prop/_config.js

@@ -0,0 +1,5 @@
+export default {
+	html: `
+		<p>undefined</p>
+	`
+};

test/runtime/samples/component-slot-let-missing-prop/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	import Foo from './Foo.svelte';
+	import Bar from './Bar.svelte';
+
+	const things = { '1': 'one' };
+</script>
+
+<Foo let:id>
+	<Bar thing={things[id]}/>
+</Foo>