fix: rootScreen null 시 MainActivity로 이동하도록 변경

- COURSE_DISCOVER와 null을 묶어서 navigateToMainScreen() 처리
- 딥링크 등 rootScreen 없는 진입 시에도 안전하게 홈으로 복귀

Author: unam98

app/src/main/java/com/runnect/runnect/presentation/detail/CourseDetailActivity.kt

@@ -407,10 +407,12 @@ class CourseDetailActivity :
             COURSE_STORAGE_SCRAP -> lifecycleScope.launch {
                 screenRefreshEventBus.emit(ScreenRefreshEvent.RefreshStorageScrap)
             }
-            COURSE_DISCOVER -> setActivityResult<MainActivity>()
             COURSE_DISCOVER_SEARCH -> setActivityResult<DiscoverSearchActivity>()
             MY_PAGE_UPLOAD_COURSE -> setActivityResult<MyUploadActivity>()
-            null -> {}
+            COURSE_DISCOVER, null -> {
+                navigateToMainScreen()
+                return
+            }
         }
 
         finish()