fix: do not render class when it is undefined or null

[evanryuu]

This PR is related to #3173.

Since normalizeStyle method can return undefined, I was just thinking why not make normalizeClass also able to return undefined? In my personal opinion, this seems more consistent.

With this PR, the props in the example in #3173 should be rendered consistently as below:

string

undefined

null

[baiwusanyu-c]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

suite	result	latest scheduled
nuxt	✅ success	✅ success
pinia	✅ success	✅ success
quasar	✅ success	❌ failure
router	✅ success	✅ success
test-utils	✅ success	✅ success
vant	❌ failure	✅ success
vite-plugin-vue	✅ success	✅ success
vitepress	✅ success	✅ success
vue-i18n	✅ success	✅ success
vue-macros	❌ failure	❌ failure
vuetify	✅ success	✅ success
vueuse	✅ success	✅ success
vue-simple-compiler	✅ success	✅ success

[baiwusanyu-c]

LGTM, but there seems to be a bit of a ‘breaking change’

[evanryuu]

LGTM, but there seems to be a bit of a ‘breaking change’

Thank you for your reply!

I looked into the failed tests, and I can't quite figure out why this PR caused the tests to fail😇 My apologies.

[gokaybiz]

For Vant, it probably fails because of the data type here.
https://github.com/youzan/vant/blob/fae9a236e2d17bef63ab76527cdd5dc3309cb154/packages/vant/src/tab/Tab.tsx#L96

[evanryuu]

For Vant, it probably fails because of the data type here. youzan/vant@fae9a23/packages/vant/src/tab/Tab.tsx#L96

Thank you! It does look like Vant relies on normalizeClass returning string only.

[evanryuu]

I didn't realize I accidentally deleted the source repo until now. Apologies for my mistakes☹️ But I also believe there could be better solutions. Hope someone else could solve this more elegantly.