refactor: logic seperation for related-subject/

Author: Gslmao

src/lib/services/subject.ts

@@ -1,19 +1,20 @@
 import { connectToDatabase } from "@/lib/database/mongoose";
-import { Course } from "@/db/course";
 import { IRelatedSubject } from "@/interface";
 import { escapeRegExp } from "@/lib/utils/regex";
+import { Course } from "@/db/course";
 import RelatedSubject from "@/db/relatedSubjects";
 
 export async function getCourseList(){
 	await connectToDatabase();
 	return await Course.find().lean();
 }
+
 export async function getRelatedSubjects(subject: string) {
 	await connectToDatabase();
 	const escapedSubject = escapeRegExp(subject);
 	const subjects: IRelatedSubject[] = await RelatedSubject.find({
 		subject: { $regex: new RegExp(`${escapedSubject}`, "i") },
 	});
 
-	return subjects[0]?.related_subjects ?? [];
-}
+	return subjects[0]?.related_subjects || [];
+}