fix: select first enabled option by default when initial value is und…

…efined

Fixes an unintended regression introduced in sveltejs#6170.

Resolves sveltejs#7041

src/runtime/internal/dom.ts

@@ -557,7 +557,9 @@ export function select_option(select, value) {
 		}
 	}
 
-	select.selectedIndex = -1; // no option should be selected
+	if (value !== undefined) {
+		select.selectedIndex = -1; // no option should be selected
+	}
 }
 
 export function select_options(select, value) {

test/runtime/samples/binding-select-initial-value-undefined/_config.js

@@ -5,6 +5,7 @@ export default {
 		<p>selected: a</p>
 
 		<select>
+			<option disabled='' value='x'>x</option>
 			<option value='a'>a</option>
 			<option value='b'>b</option>
 			<option value='c'>c</option>
@@ -18,7 +19,8 @@ export default {
 		const select = target.querySelector('select');
 		const options = [...target.querySelectorAll('option')];
 
+		// first enabled option should be selected
 		assert.equal(select.value, 'a');
-		assert.ok(options[0].selected);
+		assert.ok(options[1].selected);
 	}
 };

test/runtime/samples/binding-select-initial-value-undefined/main.svelte

@@ -5,6 +5,7 @@
 <p>selected: {selected}</p>
 
 <select bind:value={selected}>
+	<option disabled>x</option>
 	<option>a</option>
 	<option>b</option>
 	<option>c</option>