Fix classNames runtime helper for undefined and unmapped classes

[deanmarano]

Summary

• Guard against undefined/null/empty string being passed to classNames() in the runtime helper — previously threw TypeError: Cannot read properties of undefined (reading 'trim')
• Drop unmapped class names instead of passing them through — the ?? fallback in classNames() was passing unknown classes (e.g. global utility classes like .is-hidden) to the DOM, which changed behavior from v2 where they were silently dropped. This caused issues like CSS transitions breaking when global display: none classes were unexpectedly applied.

Test plan

• vitest run passes for runtime tests (3/3)
• CI green on fork: Handle undefined classes in classNames runtime helper deanmarano/ember-css-modules#2
• All tests passing in consuming app

See also: #318