fix(compiler-core/v-on): only cache empty handler when the option is …

…used

fix #1716

The original idea is that the empty handler can be cached in all cases,
but this contradicts with the assumption that `hasCachedProps` check in
the static hoist transform can simply return `false` in the browser
build.

packages/compiler-core/src/transforms/vOn.ts

@@ -68,7 +68,7 @@ export const transformOn: DirectiveTransform = (
  if (exp && !exp.content.trim()) {
  exp = undefined
  }
- let isCacheable: boolean = !exp
+ let isCacheable: boolean = context.cacheHandlers && !exp
  if (exp) {
  const isMemberExp = isMemberExpression(exp.content)
  const isInlineStatement = !(isMemberExp || fnExpRE.test(exp.content))