fix: Jpa AuditorAware에서의 NPE 수정

src/main/kotlin/roomescape/common/config/JpaConfig.kt

@@ -18,8 +18,12 @@ class JpaConfig {
 
 class MdcAuditorAware : AuditorAware<Long> {
     override fun getCurrentAuditor(): Optional<Long> {
-        val memberIdStr = MDC.get(MDC_MEMBER_ID_KEY)
+        val memberIdStr: String? = MDC.get(MDC_MEMBER_ID_KEY)
 
-        return Optional.ofNullable(memberIdStr.toLongOrNull())
+        if (memberIdStr == null) {
+            return Optional.empty()
+        } else {
+            return Optional.of(memberIdStr.toLong())
+        }
     }
 }