refactor: add empty v-bind warnings

re vuejs#7973

Author: lyhper

src/compiler/parser/index.js

@@ -528,6 +528,14 @@ function processAttrs (el) {
         name = name.replace(bindRE, '')
         value = parseFilters(value)
         isProp = false
+        if (
+          process.env.NODE_ENV !== 'production' &&
+          value.trim().length === 0
+        ) {
+          warn(
+            `The value for a v-bind expression cannot be empty. Found in "${name}"`
+          )
+        }
         if (modifiers) {
           if (modifiers.prop) {
             isProp = true

test/unit/modules/compiler/parser.spec.js

@@ -510,6 +510,11 @@ describe('parser', () => {
     expect(ast.props[0].value).toBe('msg')
   })
 
+  it('empty v-bind expression', () => {
+    parse('<div :empty-msg=""></div>', baseOptions)
+    expect('The value for a v-bind expression cannot be empty. Found in "empty-msg"').toHaveBeenWarned()
+  })
+
   // #6887
   it('special case static attribute that must be props', () => {
     const ast = parse('<video muted></video>', baseOptions)