Set access_scopes column to string by default

[kirillplatonov]

What this PR does

Access scopes can't be nil anymore as it will trigger a sorbet error in ShopifyAPI 10+.

Parameter 'scope': Expected type T.any(String, T::Array[String]), got type NilClass
Caller: /usr/local/bundle/gems/sorbet-runtime-0.5.9878/lib/types/private/methods/call_validation.rb:117
Definition: /usr/local/bundle/gems/shopify_api-10.0.2/lib/shopify_api/auth/session.rb:52

Resolves #1411

Checklist

Before submitting the PR, please consider if any of the following are needed:

• Update CHANGELOG.md if the changes would impact users
• Update README.md, if appropriate.
• Update any relevant pages in /docs, if necessary
• For security fixes, the Disclosure Policy must be followed.

[vfonic]

I understand I'm half a year late to the party. Just wanted to understand what's the general guidance on making such changes that are not backwards compatible?
For example, if I had an app which already had the access scopes migration (without default "" and null: true), how can I upgrade to the new gem version? Do I need to manually create a new migration adding default "" and null: false?

Is there any reason why this isn't the default guidance/workflow?

Also, unrelated to this particular issue, I strongly believe that @kirillplatonov should be paid for his work on Shopify projects. @nelsonwittwer, you're getting paid for your work, correct?
Why would @kirillplatonov volunteer to upgrade your infrastructure and the only benefit he'd get is to not have to fork the repo to have his fixes in? ...but I guess that's not the status quo, it's expected from outside developers to contribute for free... (that's why I stopped contributing)
https://github.com/Shopify/shopify_app/pulls?q=author%3Akirillplatonov

[vfonic]

I just ran rails generate shopify_app and realized that these changes were not applied. It's been half a year since @kirillplatonov created this PR and more than 3 months since the PR was merged into main. What is going on?

EDIT: Not sure what the issue was, but bundle update shopify_app would not update shopify_app gem from 21.2.0 to 21.5.0. I tried specifying version restriction in several different ways, even removing '~> 21.2'. The only thing that worked was explicitly setting gem 'shopify_app', '~> 21.5'.