Resolve "Search query without results leads to error"

Closes #179 (closed)

src/main/java/nl/tudelft/tam/service/CourseService.java

@@ -17,6 +17,7 @@
  */
 package nl.tudelft.tam.service;
 
+import java.util.Collections;
 import java.util.List;
 
 import javax.validation.constraints.NotNull;
@@ -67,6 +68,8 @@ public class CourseService {
 	 * @return     The list of course details
 	 */
 	public List<CourseDetailsDTO> getCoursesById(List<Long> ids) {
+		if (ids.isEmpty())
+			return Collections.emptyList();
 		return courseApi.getAllCoursesById(ids).collectList().block();
 	}