refactor : 쪽지조회 테스트케이스 잘못 작성된 부분 수정

Author: donsonioc2010

src/test/java/com/nooblol/user/service/impl/LetterServiceImplTest.java

@@ -211,7 +211,8 @@ void getLetterListByUserId_WhenIsNotExistsLetterByDbTypeTo_ThenReturnEmptyLetter
           .build();
 
       //mock
-      when(letterMapper.selectLetterListByUserIdAndTypeTo(requestDto)).thenReturn(null);
+      when(letterMapper.selectLetterListByUserIdAndTypeTo(requestDto))
+          .thenReturn(new ArrayList<LetterDto>());
 
       //when
       List<LetterDto> result = letterService.getLetterListByUserId(requestDto);
@@ -257,7 +258,8 @@ void getLetterListByUserId_WhenIsNotExistsLetterByDbTypeFrom_ThenReturnEmptyLett
           .build();
 
       //mock
-      when(letterMapper.selectLetterListByUserIdAndTypeFrom(requestDto)).thenReturn(null);
+      when(letterMapper.selectLetterListByUserIdAndTypeFrom(requestDto))
+          .thenReturn(new ArrayList<LetterDto>());
 
       //when
       List<LetterDto> result = letterService.getLetterListByUserId(requestDto);