web/elements: fix error with blank SearchSelect elements in forms

closes #2469

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

web/src/elements/SearchSelect.ts

@@ -40,7 +40,7 @@ export class SearchSelect<T> extends LitElement {
     renderElement!: (element: T) => string;
 
     @property({ attribute: false })
-    value!: (element: T) => unknown;
+    value!: (element: T | undefined) => unknown;
 
     @property({ attribute: false })
     selected!: (element: T) => boolean;