Q:

call QuickAction in test class

In class  implements QuickAction.QuickActionDefaultsHandler and below method is just blank need to cover in test class which parameter I can pass for cover this.
mailmsg.onInitDefaults('whats need which parameter Enter');

global void onInitDefaults(QuickAction.QuickActionDefaults[] defaults) {

}

A:

You can do something like
List<QuickAction.QuickActionDefaults> defaults = new List<QuickAction.QuickActionDefaults>();
mailmsg.onInitDefaults(defaults );

This will cover the blank method.

