incubator-devlake: `issues.parent_issue_id` should be null rather than empty string if not exists

You can convert the

ParentIssueId string gorm:"type:varchar(255)"

to

ParentIssueId *string gorm:"type:varchar(255)"

Then I think you can make pass the nil

@mindlesscloud I’ll give this a try today, will let you know

@deepto98 Thanks, happy coding

@ppborah Done

Please go ahead @deepto98 😃

Can I pick this up?

Apologies for the delay @narrowizard, but I am working on it

Here you are, happy coding