如何将 IEnumerable 返回<T>为空

我有下面的ActivitiesDB.cs，它是一个DBEntity类。

public class ActivitiesDB
{
public IEnumerable<Activity> Activities { get; set; }
}
public class Activity
{
public Guid ActivityId { get ;set; }
public DateTime ActivityDateTime { get; set; }
public DateTime UpdatedDateTime { get; set; }
public IEnumerable<MetaData> MetaData { get; set; }
}
public class MetaData
{
public string Name { get; set; }
public string Value { get; set; }
}

DB中的样本数据(我在后台使用CosmosDb(：

"Activities": [
{
"ActivityId": "4f436d11-7a61-4e7f-b0fb-0f0d5bb6be76",
"ActivityDateTime": "2021-04-30T09:35:35.32Z",
"UpdatedDateTime": "2021-05-03T07:45:21.5115588Z",
"Metadata": [
{
"Name": "name",
"Value": "value"
}
]
},
{
"ActivityId": "659e62c5-4408-42b4-8719-b8c577221d22",
"ActivityDateTime": "2021-04-30T09:35:35.32Z",
"UpdatedDateTime": "2021-05-03T07:45:21.5115588Z",
"Metadata": null
}
]

为了显示数据，我有下面的DisplayActivtiesResponse.cs类：

public class DisplayActivtiesResponse
{
public IEnumerable<DisplayActivity> DisplayActivties{ get; set; }
}
public class DisplayActivity
{
public Guid ActivityId { get ;set; }
public DateTime ActivityDateTime { get; set; }
public DateTime UpdatedDateTime { get; set; }
public IEnumerable<MetaData> MetaData { get; set; }
}
public class MetaData
{
public string Name { get; set; }
public string Value { get; set; }
}

以下是从DB获取数据的代码：

public async Task<DisplayActivtiesResponse> DisplayActivityAsync(Guid activityId)
{
var feedIterator = this.cosmosReviewRequestContainer.GetItemLinqQueryable<ActivitiesDB>()
.Where(q => q.Activities.Where(x => x.ActivityId == activityId)
.ToFeedIterator();
var requestModel = new ActivitiesDB();
while (feedIterator.HasMoreResults)
{
var items = await feedIterator.ReadNextAsync().ConfigureAwait(false);
requestModel = items.FirstOrDefault();
}
return new DisplayActivtiesResponse
{
DisplayActivties = mapper.Map<IEnumerable<DisplayActivity>>(requestModel);
};
}

MappingProfile.cs:

public class MappingProfile : Profile
{
public MappingProfile()
{
this.CreateMap<Activity, DisplayActivity>().ReverseMap();
}
}

现在，当我从数据库获取数据时，我观察到如果元数据为null，那么我得到的是空数组[]，而不是null。我仍然不知道我到底错过了什么。请帮我做这件事。