[5.x] Ensure container option isn't removed from Asset fields

[duncanmcclean]

In #9685, we made it so default values for fieldtype configs are stripped out of the config array, slimming down blueprint & fieldset YAML files.

However, this change has caused an unintentional side affect.....

• When your site only has one asset container, the default value on Asset fields will be that of that asset container.
  • This means that no container is saved into the blueprint/fieldset's YAML file.
• In the future, if you decide to create an additional asset container, because there's no default container to rely on (there's only a default when a single asset container is configured) and no explicit container is set on the fields, you'll see a lot of An asset container has not been configured errors

This can be easily fixed by ensuring the container config option is always saved, so when an additional asset container is created, existing Asset fields use the initial asset container since that's what's been set.

Fixes #10050.

[jasonvarga]

Thank you but I think there is a bigger issue with how the defaults are being stripped out. I'm trying to resolve it in #10059