feat: add nullability annotations to findById method in ModificationRepository

src/main/java/com/onixbyte/deltaforceguide/repository/ModificationRepository.java

@@ -1,6 +1,7 @@
 package com.onixbyte.deltaforceguide.repository;
 
 import com.onixbyte.deltaforceguide.domain.entity.Modification;
+import org.jspecify.annotations.NonNull;
 import org.springframework.data.domain.Page;
 import org.springframework.data.domain.Pageable;
 import org.springframework.data.jpa.repository.EntityGraph;
@@ -23,7 +24,8 @@ public interface ModificationRepository extends JpaRepository<Modification, Long
 
     @Override
     @EntityGraph(attributePaths = {"firearm"})
-    Optional<Modification> findById(Long id);
+    @NonNull
+    Optional<Modification> findById(@NonNull Long id);
 
     @Query(value = """
             SELECT * FROM modification m