fix unsetting some values

- Category prefix
- category in motion multiSelect

client/src/app/site/motions/components/category-list/category-list.component.ts

@@ -153,9 +153,7 @@ export class CategoryListComponent extends BaseViewComponent implements OnInit {
     public async onSaveButton(viewCategory: ViewCategory): Promise<void> {
         if (this.updateForm.dirty && this.updateForm.valid) {
             const cat: Partial<Category> = { name: this.updateForm.get('name').value };
-            if (this.updateForm.get('prefix').value) {
-                cat.prefix = this.updateForm.get('prefix').value;
-            }
+            cat.prefix = this.updateForm.get('prefix').value || '';
             await this.repo.update(cat, viewCategory);
             this.updateForm.markAsPristine();
         }

client/src/app/site/motions/services/motion-multiselect.service.ts

@@ -147,7 +147,7 @@ export class MotionMultiselectService {
         if (selectedChoice) {
             for (const motion of motions) {
                 await this.repo.update(
-                    { category_id: selectedChoice.action ? 0 : (selectedChoice.items as number) },
+                    { category_id: selectedChoice.action ? null : (selectedChoice.items as number) },
                     motion
                 );
             }