warn empty or missing template element (fix #4171)

Author: yyx990803

src/core/instance/lifecycle.js

@@ -44,7 +44,7 @@ export function lifecycleMixin (Vue: Class<Component>) {
       vm.$options.render = emptyVNode
       if (process.env.NODE_ENV !== 'production') {
         /* istanbul ignore if */
-        if (vm.$options.template) {
+        if (vm.$options.template && vm.$options.template.charAt(0) !== '#') {
           warn(
             'You are using the runtime-only build of Vue where the template ' +
             'option is not available. Either pre-compile the templates into ' +

src/entries/web-runtime-with-compiler.js

@@ -34,6 +34,13 @@ Vue.prototype.$mount = function (
       if (typeof template === 'string') {
         if (template.charAt(0) === '#') {
           template = idToTemplate(template)
+          /* istanbul ignore if */
+          if (process.env.NODE_ENV !== 'production' && !template) {
+            warn(
+              `Template element not found or is empty: ${options.template}`,
+              this
+            )
+          }
         }
       } else if (template.nodeType) {
         template = template.innerHTML