test: add test for runtime-dom/modules/class (#75)

packages/runtime-dom/src/modules/class.ts

@@ -2,7 +2,10 @@ import { ElementWithTransition } from '../components/Transition'
 
 // compiler should normalize class + :class bindings on the same element
 // into a single binding ['staticClass', dynamic]
-export function patchClass(el: Element, value: string, isSVG: boolean) {
+export function patchClass(el: Element, value: string | null, isSVG: boolean) {
+  if (value == null) {
+    value = ''
+  }
   // directly setting className should be faster than setAttribute in theory
   if (isSVG) {
     el.setAttribute('class', value)