feat: code group provides logical fallthrough behavior

packages/@vuepress/theme-default/global-components/CodeGroup.vue

@@ -12,6 +12,10 @@
       </button>
     </div>
     <slot />
+    <pre
+      v-if="codeTabs.length < 1"
+      class="pre-blank"
+    >// Make sure to add code blocks to your code group</pre>
   </div>
 </template>
 
@@ -21,7 +25,7 @@ export default {
   data () {
     return {
       codeTabs: [],
-      activeCodeTabIndex: 0
+      activeCodeTabIndex: -1
     }
   },
   watch: {
@@ -33,10 +37,20 @@ export default {
     }
   },
   mounted () {
-    this.codeTabs = this.$slots.default.filter(slot => Boolean(slot.componentOptions)).map(slot => ({
-      title: slot.componentOptions.propsData.title,
-      elm: slot.elm
-    }))
+    this.codeTabs = (this.$slots.default || []).filter(slot => Boolean(slot.componentOptions)).map((slot, index) => {
+      if (slot.componentOptions.propsData.active === '') {
+        this.activeCodeTabIndex = index
+      }
+
+      return {
+        title: slot.componentOptions.propsData.title,
+        elm: slot.elm
+      }
+    })
+
+    if (this.activeCodeTabIndex === -1 && this.codeTabs.length > 0) {
+      this.activeCodeTabIndex = 0
+    }
   },
   methods: {
     changeCodeTab (index) {
@@ -66,4 +80,7 @@ export default {
   .theme-code-group__nav-tab-active {
     border-bottom: #42b983 1px solid;
   }
+  .pre-blank {
+    color: #42b983;
+  }
 </style>