Fix bug with clearing topics input on match page

* When clearing the input field, topics is temporarily set to null before the onClear event is emitted
* This causes hasQuestionsValidator to reference a null variable
* The fix is to add an additional Validators.required to prevent hasQuestionsValidator from firing before topics is set to an empty array

frontend/src/app/matching/matching.component.ts

@@ -62,7 +62,7 @@ export class MatchingComponent implements OnInit {
         this.fetchTopics();
         this.matchForm = new FormGroup(
             {
-                topics: new FormControl([], [Validators.minLength(1)]),
+                topics: new FormControl([], [Validators.required, Validators.minLength(1)]),
                 difficulty: new FormControl<Difficulty | null>(null, [Validators.required]),
             },
             {