fix(select): fix undefined class on select container

closes #4184

Author: rschmukler

src/components/select/select.js

@@ -344,7 +344,9 @@ function SelectDirective($mdSelect, $mdUtil, $mdTheming, $mdAria, $rootElement,
         selectEl.data('$mdSelectController', mdSelectCtrl);
         selectScope = scope.$new();
         $mdTheming.inherit(selectContainer, element);
-        selectContainer[0].setAttribute('class', selectContainer[0].getAttribute('class') + ' ' + element.attr('md-container-class'));
+        if (element.attr('md-container-class')) {
+          selectContainer[0].setAttribute('class', selectContainer[0].getAttribute('class') + ' ' + element.attr('md-container-class'));
+        }
         selectContainer = $compile(selectContainer)(selectScope);
         selectMenuCtrl = selectContainer.find('md-select-menu').controller('mdSelectMenu');
       }