Update RequestService.java reject function to fix potential nullpointer...

Update RequestService.java reject function to fix potential nullpointer exception if request doesn't have assistant yet.

src/main/java/nl/tudelft/ewi/queue/service/RequestService.java

@@ -199,7 +199,7 @@ public class RequestService {
      * @param request
      */
     public void reject(User assistant, Request request, String comment, String commentForStudent) {
-        if (!request.getAssistant().equals(assistant) && !(assistant.teaches(request.getAssignment().getCourse()) || assistant.isAdmin())) {
+        if (!assistant.equals(request.getAssistant()) && !(assistant.teaches(request.getAssignment().getCourse()) || assistant.isAdmin())) {
             throw new IllegalArgumentException("You cannot reject a request that is not assigned to you.");
         }
         request.addEvent(new RequestRejectedEvent(request, assistant,LocalDateTime.now(), comment, commentForStudent));