refactor: improve quiz retrieval logic in Quiz schema

- Enhanced the quiz retrieval logic to ensure a valid random index is calculated based on the number of available quizzes, preventing potential errors when no quizzes are present.
- Updated the logic to use 'nrOfQuestions' from the quiz object for slicing the quizzes array, ensuring consistent behavior in quiz selection.
- Improved code clarity by refining comments related to the quiz selection process.

src/Quiz/quiz.schema.ts

@@ -1,5 +1,5 @@
 import crypto from 'crypto'
-import { Inject, Injectable } from '@nestjs/common'
+import { Inject, Injectable, Logger } from '@nestjs/common'
 import { AnswerType, Role } from '@prisma/client'
 import { QuestionType } from '@prisma/client'
 import { Pothos, PothosRef, PothosSchema, SchemaBuilderToken } from '@smatch-corp/nestjs-pothos'
@@ -195,8 +195,12 @@ export class QuizSchema extends PothosSchema {
               },
             })
             // get amount of questions using nrOfQuestions and random index based on random
-            const randomIndex = Math.floor(random * (quizzes[0]?.nrOfQuestions ?? 1))
-            return quizzes.slice(randomIndex, randomIndex + (quizzes[0]?.nrOfQuestions ?? 1))
+            const randomIndex = quizzes.length > 0 
+              ? Math.floor(random * quizzes.length) 
+              : 0
+            const nrOfQuestions = quizzes[0]?.nrOfQuestions ?? 1
+            const result = quizzes.slice(randomIndex, randomIndex + nrOfQuestions)
+            return result
           }
 
           // use case 2: center mentor or center owner