fix(compiler): warn against v-bind with empty attribute value

ref: https://github.com/vuejs/vue-next/issues/1128#issuecomment-624647434

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

@@ -10,7 +10,7 @@ import { CAMELIZE } from '../runtimeHelpers'
 export const transformBind: DirectiveTransform = (dir, node, context) => {
   const { exp, modifiers, loc } = dir
   const arg = dir.arg!
-  if (!exp) {
+  if (!exp || (exp.type === NodeTypes.SIMPLE_EXPRESSION && !exp.content)) {
     context.onError(createCompilerError(ErrorCodes.X_V_BIND_NO_EXPRESSION, loc))
   }
   // .prop is no longer necessary due to new patch behavior