message: Remove empty select option from code fixes dropdown
diff: <mat-form-field appearance="fill">
  <mat-label>Correct Fix</mat-label>
  <select matNativeControl (change)="changeFix($event)">
  - <option value=""></option>
  <option *ngFor="let fix of randomFixes; index as i" [value]="i"
  [selected]="selectedFix === i">Fix {{ i+1 }}</option>
  </select>
  