fix(tabs): adds proper detection for bodyless tabs

Closes #4120

Author: Robert Messerle

src/components/tabs/js/tabDirective.js

@@ -67,12 +67,11 @@ function MdTab () {
         label = angular.element('<md-tab-label></md-tab-label>');
         if (attr.label) label.text(attr.label);
         else label.append(element.contents());
-      }
-
-      if (body.length == 0) {
-        var contents = element.contents().detach();
-        body         = angular.element('<md-tab-body></md-tab-body>');
-        body.append(contents);
+        if (body.length == 0) {
+          var contents = element.contents().detach();
+          body         = angular.element('<md-tab-body></md-tab-body>');
+          body.append(contents);
+        }
       }
 
       element.append(label);