message: Make conversion of category/tag into i18n key null-safe
diff: <mat-button-toggle class="category-toggle" appearance="legacy" *ngFor="let category of availableChallengeCategories"
  (change)="toggleShowChallengeCategory(category)"
  [checked]="displayedChallengeCategories.includes(category)" [disabled]="restrictToTutorialsFirst && !allTutorialsCompleted"
  - [matTooltip]="restrictToTutorialsFirst && !allTutorialsCompleted ? ('INFO_FULL_CHALLENGE_MODE' | translate: {num: challenges.length}) : ('CATEGORY_' + category.toUpperCase().split(' ').join('_') + '_DESCRIPTION' | translate)" matTooltipPosition="above">
  - {{('CATEGORY_' + category.toUpperCase().split(' ').join('_') | translate)}}
  + [matTooltip]="restrictToTutorialsFirst && !allTutorialsCompleted ? ('INFO_FULL_CHALLENGE_MODE' | translate: {num: challenges.length}) : ('CATEGORY_' + category?.toUpperCase().split(' ').join('_') + '_DESCRIPTION' | translate)" matTooltipPosition="above">
  + {{('CATEGORY_' + category?.toUpperCase().split(' ').join('_') | translate)}}
  </mat-button-toggle>
  <button class="mat-button" (click)="toggleAllChallengeCategory()" [disabled]="restrictToTutorialsFirst && !allTutorialsCompleted">{{(this.toggledMajorityOfCategories ? "BTN_HIDE_ALL" : "BTN_SHOW_ALL") | translate}}</button>
  <ng-container matColumnDef="category">
  <mat-header-cell *matHeaderCellDef fxShow fxHide.lt-lg translate>LABEL_CATEGORY</mat-header-cell>
  <mat-cell *matCellDef="let challenge" fxShow fxHide.lt-lg>
  - {{('CATEGORY_' + challenge.category.toUpperCase().split(' ').join('_') | translate)}}
  + {{('CATEGORY_' + challenge.category?.toUpperCase().split(' ').join('_') | translate)}}
  </mat-cell>
  </ng-container>
  <mat-cell *matCellDef="let challenge" fxShow fxHide.lt-lg>
  <span *ngIf="challenge.tags">
  <mat-chip-list *ngFor="let tag of challenge.tags.split(',').sort()">
  - <mat-chip [matTooltip]="('TAG_' + tag.toUpperCase().split(' ').join('_') + '_DESCRIPTION' | translate)">
  - {{('TAG_' + tag.toUpperCase().split(' ').join('_') | translate)}}
  + <mat-chip [matTooltip]="('TAG_' + tag?.toUpperCase().split(' ').join('_') + '_DESCRIPTION' | translate)">
  + {{('TAG_' + tag?.toUpperCase().split(' ').join('_') | translate)}}
  </mat-chip>
  </mat-chip-list>
  </span>
  