feat(aio): display “code sample missing” message when no code sample

Not having code in a `<code-example>` or `<code-tabs>` is presumed to be an error.
Hides the copy button as well.

aio/src/app/embedded/code/code.component.ts

@@ -25,7 +25,7 @@ const copiedLabel = 'Copied!';
   template: `
 
     <pre class="prettyprint lang-{{language}}">
-      <button class="material-icons copy-button" (click)="doCopy()">content_copy</button>
+      <button *ngIf="code" class="material-icons copy-button" (click)="doCopy()">content_copy</button>
       <code class="animated fadeIn" #codeContainer></code>
     </pre>
     `
@@ -70,9 +70,12 @@ export class CodeComponent implements OnChanges {
     private logger: Logger) {}
 
   ngOnChanges() {
-    if (!this.code) { return; }
+    this.code = this.code && this.code.trim();
 
-    this.code = this.code.trim();
+    if (!this.code) {
+      this.setCodeHtml('<p class="code-missing">The code sample is missing.</p>');
+      return;
+    }
 
     const linenums = this.getLinenums();