fix: reset key filter

src/app/modules/songs/song-list/filter/filter.component.html

@@ -18,6 +18,7 @@
     <mat-form-field appearance="outline">
       <mat-label>Tonart</mat-label>
       <mat-select formControlName="key">
+        <mat-option [value]="null">- kein Filter -</mat-option>
         <mat-option *ngFor="let key of keys" [value]="key">{{key|key}}</mat-option>
       </mat-select>
     </mat-form-field>