Handle dataclasses with unitialized fields in pretty printing

[CollinHeist]

Fixes #3417

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

This is a fix for #3417. If the field has no defined attribute, then the default value should be printed instead, rather than raising an uncaught AttributeError.

[willmcgugan]

This probably isn't the fix. It can give the false impression that the attribute exists. Better to omit it entirely IMO.

[CollinHeist]

Sure, I've made that change.

[willmcgugan]

Thanks, but I've gone with a slightly different solution... #3472