refactor: improve handling of optional CustomerID in project models and DTOs

backend/internal/api/handlers/project_handler.go

@@ -148,13 +148,17 @@ func (h *ProjectHandler) DeleteProject(c *gin.Context) {
 // Helper functions for DTO conversion
 
 func convertProjectToDTO(project *models.Project) dto.ProjectDto {
-	customerId := project.CustomerID.String()
+	var customerIdPtr *string
+	if project.CustomerID != nil {
+		customerIdStr := project.CustomerID.String()
+		customerIdPtr = &customerIdStr
+	}
 	return dto.ProjectDto{
 		ID:         project.ID.String(),
 		CreatedAt:  project.CreatedAt,
 		UpdatedAt:  project.UpdatedAt,
 		Name:       project.Name,
-		CustomerID: &customerId,
+		CustomerID: customerIdPtr,
 	}
 }
 
@@ -182,13 +186,13 @@ func convertUpdateProjectDTOToModel(dto dto.ProjectUpdateDto, id types.ULID) (mo
 
 	if dto.CustomerID.Valid {
 		if dto.CustomerID.Value == nil {
-			update.CustomerID = nil
+			update.CustomerID = types.Null[types.ULID]()
 		} else {
 			customerID, err := types.ULIDFromString(*dto.CustomerID.Value)
 			if err != nil {
 				return models.ProjectUpdate{}, fmt.Errorf("invalid customer ID: %w", err)
 			}
-			update.CustomerID = &customerID
+			update.CustomerID = types.NewNullable(customerID)
 		}
 	}