fix ssr dropping bound value 0

Number 0 when used in the value of a binding was
ignored by the server side rendering.

src/runtime/internal/ssr.ts

@@ -128,7 +128,7 @@ export function create_ssr_component(fn) {
 }
 
 export function add_attribute(name, value, boolean) {
-	if (value == null || (boolean && !value)) return '';
+	if (value == null || (boolean && !value && value !== 0)) return '';
 	return ` ${name}${value === true ? '' : `=${typeof value === 'string' ? JSON.stringify(escape(value)) : `"${value}"`}`}`;
 }
 

test/server-side-rendering/samples/bindings-zero/_expected.html

@@ -0,0 +1,2 @@
+<input value='0' type='number'>
+<input value='0' type='range'>

test/server-side-rendering/samples/bindings-zero/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	export let zero = 0;
+</script>
+
+<input bind:value={zero} type="number">
+<input bind:value={zero} type="range">