Empty overflow action is changed to BLOCK

When configuring overflow action on async log handler, it is possible to select an empty action. This is automatically changed to BLOCK upon saving. In CLI it is not possible to set an empty action, please remove it from the list of actions.

NOTE: it is possible to create a new async handler with no action also in CLI, which is automatically set to BLOCK. This behavior is OK since the default value is set when no value is set by user. However, when editing an existing async handler, it should not be possible to set an emtpy value.