Avoid a missing JCheckBoxItem checkmark for boolean actions without an icon

[eirikbakke]

In menu bars, a boolean action will become a JCheckBoxMenuItem which can work with or without a custom icon. If the action has no icon of its own, the LAF will provide a checkmark icon instead, which is good. But due to a bug in org.openide.awt.Actions, a blank icon is used instead. Fix this so the checkmark properly appears in this case.

(Mostly relevant for NetBeans Platform apps; I haven't seen this case appear in the NetBeans IDE.)