fix: rowPassword 빈 문자열로 변경

Author: zizi0308

src/main/java/org/dfbf/soundlink/domain/user/service/KakaoAuthService.java

@@ -106,7 +106,7 @@ private User registerNewKakaoUser(KakaoUserDTO kakaoUser, String nickname) {
                 .socialId(Long.valueOf(kakaoUser.id()))
                 .socialType(SocialType.KAKAO)
                 .loginId("kakao_" + kakaoUser.id()) // 카카오 ID 기반 로그인 ID 생성
-                .password(passwordEncoder.encode(null))
+                .password(passwordEncoder.encode(""))
                 .email(kakaoUser.kakao_account().email())
                 .build();
         return userRepository.save(newUser);