update issue update root id

app/services/api/v1/issues/update_service.rb

@@ -77,12 +77,8 @@ class Api::V1::Issues::UpdateService < ApplicationService
       @updated_issue.pm_project_id = @target_pm_project_id unless @target_pm_project_id.nil?
       @updated_issue.pm_sprint_id = @pm_sprint_id unless @pm_sprint_id.nil?
       @updated_issue.pm_issue_type = @pm_issue_type unless @pm_issue_type.nil?
-      @updated_issue.root_id = if @root_id.nil? || @root_id.try(:zero?)
-                                 nil
-                               else
-                                 @root_id
-                               end
-
+      @updated_issue.root_id = @root_id unless @root_id.nil? #不为 nil的时候更新
+      @updated_issue.root_id = nil if @root_id.try(:zero?) #为 0 的时候设置为 nil
       @updated_issue.time_scale = @time_scale unless @time_scale.nil?
 
       @updated_issue.updated_on = Time.now