use default action button class when fill and color props aren't given

awx/ui/client/lib/components/action/action-button.partial.html

@@ -1,3 +1,3 @@
-<button class="btn at-Button{{ vm.fill }}--{{ vm.color }}">
+<button class="btn at-Button{{ vm.fill }}{{ vm.color ? '--' + vm.color : '' }}">
     <ng-transclude></ng-transclude>
 </button>