refactor: getUser 에서 유저가 발견되지 않으면 오류 발생 (#13)

src/main/java/com/github/winkathon/lingo/domain/user/exception/UserNotFoundException.java

@@ -0,0 +1,13 @@
+package com.github.winkathon.lingo.domain.user.exception;
+
+import org.springframework.http.HttpStatus;
+
+import com.github.winkathon.lingo.common.api.exception.ApiException;
+
+public class UserNotFoundException extends ApiException {
+
+    public UserNotFoundException() {
+
+        super(HttpStatus.NOT_FOUND, "유저를 찾을 수 없습니다.");
+    }
+}

src/main/java/com/github/winkathon/lingo/domain/user/service/UserService.java

@@ -11,6 +11,7 @@
 import com.github.winkathon.lingo.domain.user.dto.request.ChangePasswordRequest;
 import com.github.winkathon.lingo.domain.user.dto.response.UserListResponse;
 import com.github.winkathon.lingo.domain.user.dto.response.UserResponse;
+import com.github.winkathon.lingo.domain.user.exception.UserNotFoundException;
 import com.github.winkathon.lingo.domain.user.repository.UserRepository;
 import com.github.winkathon.lingo.domain.user.schema.User;
 
@@ -45,7 +46,7 @@ public UserResponse getUser(String userId) {
 
                     return user1;
                 })
-                .orElse(null);
+                .orElseThrow(UserNotFoundException::new);
 
         return UserResponse.builder()
                 .user(user)