MessageBox: fix multiple instances with duplicate title (#10126)

packages/message-box/src/main.js

@@ -1,5 +1,5 @@
 const defaults = {
-  title: undefined,
+  title: null,
   message: '',
   type: '',
   showInput: false,

packages/message-box/src/main.vue

@@ -10,7 +10,7 @@
       :aria-label="title || 'dialog'"
     >
       <div class="el-message-box" :class="[customClass, center && 'el-message-box--center']">
-        <div class="el-message-box__header" v-if="title !== undefined">
+        <div class="el-message-box__header" v-if="title !== null">
           <div class="el-message-box__title">
             <div class="el-message-box__status" :class="[ typeClass ]" v-if="typeClass && center"></div>
             <span>{{ title }}</span>