Fix null option being selected when it doesn't match the bound value

packages/svelte/src/internal/client/render.js

@@ -458,7 +458,7 @@ export function select_option(select, value, mounting) {
 		}
 	}
 	if (!mounting || value !== undefined) {
-		select.value = '';
+		select.selectedIndex = -1; // no option should be selected
 	}
 }
 

packages/svelte/tests/runtime-legacy/samples/binding-select-null-placeholder-2/_config.js

@@ -24,5 +24,11 @@ export default test({
 		component.foo = items[0];
 		assert.equal(options[0].selected, false);
 		assert.equal(options[1].selected, true);
+
+		component.foo = { id: 'c' }; // doesn't match an option
+		assert.equal(select.value, '');
+		assert.equal(select.selectedIndex, -1);
+		assert.equal(options[0].selected, false);
+		assert.equal(options[1].selected, false);
 	}
 });