fix(forms): remove undefined class name

[Leopoldthecoder]

Summary

If field.labelClasses is undefined, before this PR:

After this PR:

PR Checklist

• Naming & Structure: the files and package structure use the conventions outlined in the Creating a Package docs.
• Tests pass: check the output of all package unit and/or component tests.
  • If this PR is the result of a bug, test coverage was added accordingly.
• Functional: all changes do not break existing APIs, but if so, a BREAKING CHANGE commit is in place to bump the major version.
• Conventional Commits all commits follow the conventional commit standards outlined in the main README.
• Docs: includes a technically accurate README, and the docs have been updated accordingly based on the changes in this PR.