fix: fix another logic related to non-null constraints

- nullable 조건 추가로 인한 자잘한 리팩터링
- 커스텀 예외 로직 반환하도록 수정

src/main/kotlin/com/dobby/backend/application/usecase/experiment/CreateExperimentPostUseCase.kt

@@ -193,7 +193,9 @@ class CreateExperimentPostUseCase(
 
     private fun validateOnlineMatchType(input: Input){
         if(input.matchType == MatchType.ONLINE){
-            require(input.univName == null && input.region == null && input.area == null)
+            if(input.univName != null || input.region != null || input.area != null) {
+                throw ExperimentPostException(ErrorCode.EXPERIMENT_POST_INVALID_ONLINE_REQUEST)
+            }
         }
     }
 }

src/main/kotlin/com/dobby/backend/application/usecase/experiment/GetExperimentPostsUseCase.kt

@@ -47,7 +47,7 @@ class GetExperimentPostsUseCase (
         val experimentPostId: Long,
         val title: String,
         val views: Int,
-        val univName: String,
+        val univName: String?,
         val reward: String,
         val recruitStatus: Boolean,
         val durationInfo: DurationInfoOutput

src/main/kotlin/com/dobby/backend/domain/exception/ErrorCode.kt

@@ -89,5 +89,5 @@ enum class ErrorCode(
     EXPERIMENT_POST_IMAGE_SIZE_LIMIT("EP005", "Image can be uploaded maximum 3 images.", HttpStatus.BAD_REQUEST),
     EXPERIMENT_POST_RECRUIT_STATUS_ERROR("EP006", "This experiment post has already closed recruitment.", HttpStatus.BAD_REQUEST),
     EXPERIMENT_POST_CANNOT_UPDATE_DATE("EP007", "You cannot update experiment post with past experiment dates.", HttpStatus.BAD_REQUEST),
-    EXPERIMENT_POST_CANNOT_UPDATE_ALARM("EP008", "The alarmAgree for the experiment post cannot be updated.", HttpStatus.BAD_REQUEST)
+    EXPERIMENT_POST_INVALID_ONLINE_REQUEST("EP008", "univName, region, area field value must be null when MatchType is online.", HttpStatus.BAD_REQUEST),
 }

src/main/kotlin/com/dobby/backend/presentation/api/dto/response/experiment/ExperimentPostDetailResponse.kt

@@ -88,13 +88,13 @@ data class ExperimentPostDetailResponse(
     @Schema(description = "주소 응답 DTO")
     data class AddressResponse(
         @Schema(description = "학교", example = "건국대학교")
-        val univName: String,
+        val univName: String?,
 
         @Schema(description = "지역", example = "SEOUL")
-        val region: Region,
+        val region: Region?,
 
         @Schema(description = "구/군", example = "GWANGJINGU")
-        val area: Area,
+        val area: Area?,
 
         @Schema(description = "상세 주소 (nullable)", example = "1동 101호", nullable = true)
         val detailedAddress: String?