WIP: fix: set empty string as theme property default value

Author: Felipe Lang

src/main/resources/META-INF/resources/frontend/src/fc-days-of-week-selector.ts

@@ -38,7 +38,7 @@ export class DaysOfWeekSelector extends ResizeMixin(ThemableMixin(LitElement)) {
     _container!: HTMLElement;
 
     @property()
-    theme: string | null = null;
+    theme: string = '';
 
     @property()
     label: string = '';