[som] Render alerts that don't have extensions.

go/src/infra/appengine/sheriff-o-matic/elements/som-extension-build-failure.html

 <link rel="import" href="/bower_components/polymer/polymer.html">
 <link rel="import" href="/bower_components/iron-ajax/iron-ajax.html">
 <link rel="import" href="/bower_components/iron-collapse/iron-collapse.html">
 <link rel="import" href="/bower_components/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="/bower_components/iron-icons/hardware-icons.html">
 <link rel="import" href="/bower_components/iron-icons/iron-icons.html">
 <link rel="import" href="/bower_components/iron-page-url/iron-page-url.html">
 <link rel="import" href="/bower_components/paper-button/paper-button.html">
 <link rel="import" href="/bower_components/paper-card/paper-card.html">
 <link rel="import" href="/bower_components/paper-drawer-panel/paper-drawer-panel.html">
@@ skipping 143 matching lines @@
         if (this._failureCount(builder) > 1) {
           classes.push('multiple-failures');
         }
         if (this.type == 'infra-failure') {
           classes.push('infra-failure');
         }
         return classes.join(' ');
       },
 
       _haveSuspectCLs: function(extension) {
-        return !!extension.suspected_cls;
+        return extension && extension.suspected_cls;
       },
 
       _linkForTest: function(reason, testName) {
         return 'https://test-results.appspot.com/dashboards/flakiness_dashboard.html#' +
           "tests=" + encodeURIComponent(testName) +
           "&testType=" + encodeURIComponent(reason.step)
           ;
       },
 
       _linkForCL: function(cl) {
         return 'https://crrev.com/' + cl.revision;
       },
 
       _showRegressionRange: function(range) {
         return range.positions && range.positions.length > 0 &&
             range.repo != 'v8';
       },
 
       _textForCL: function(cl) {
         return '(details)';
       },
     });
   })();
   </script>
 </dom-module>