Draft: Demonstrate issue with trigger and null values

What does this MR do and why?

Minimal reproduction case for bug discovered while working on !80065 (merged)

See !80537 (merged)

How to set up and validate locally

This can be demonstrated by constructing an object and then saving it:

[1] pry> i = build(:integration, type: 'Integrations::Jira' , project: project)
=> #<Integration:0x00007f59eb8b2d78
 id: nil,
 project_id: 1,
 ...
 type_new: "Integrations::Jira",
 ... 
 >
[2] pry> i.type
=> "Integrations::Jira" # looks good!
[3] pry> i.save!
=> true                 # great!
[4] pry> i.type
=> "Integrations::Jira" # excellent!
[5] pry> i.type_new 
=> "Integrations::Jira" # perfect!
[6] pry> Integration.pluck(:type_new, :type)
=> [[nil, nil], ...] # oh no!

We can prove that this is due to the trigger thusly:

[.] pry> Integration.connection.execute 'ALTER TABLE integrations DISABLE TRIGGER "trigger_type_new_on_insert"'
[.] pry> i = build(:integration, type: 'Integrations::Jira' , project: project)
[.] pry> i.save!
[.] pry> Integration.pluck(:id, :type_new, :type)
=> [..., [12, nil, nil], [13, "Integrations::Jira", "Integrations::Jira"]]

MR acceptance checklist

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Related to #333508 (closed)