MongoDB是否按文档数组中的字段进行匹配（如果不存在）->跳过此字段的匹配

I've got collection named 'banners'. If user choosed Continent and Country the body of this document will be:

 "Continents" : [ 
     4, 
     6
 ],
 "Countries" : [ 
     "74d06620-79b1-4449-9a17-848169d514ce"
 ]

But, if user choosed all Countries in this Continent:

 "Continents" : [ 
     4, 
     6
 ],
 "Countries" : [],

How can i match in aggregation by field in document's array if it exists, if not -> skip match by this field and continue searching? Any help would be appreciated)