refactor: simplify runtime-dom class tests

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

@@ -6,15 +6,18 @@ 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)
   } else {
+    // directly setting className should be faster than setAttribute in theory
     // if this is an element during a transition, take the temporary transition
     // classes into account.
     const transitionClasses = (el as ElementWithTransition)._vtc
     if (transitionClasses) {
-      value = [value, ...transitionClasses].join(' ')
+      value = (value
+        ? [value, ...transitionClasses]
+        : [...transitionClasses]
+      ).join(' ')
     }
     el.className = value
   }