我发现了导致这个结果的真正原因，值为空，与控制器类型无关--它是在解释来自存储库函数的返回值时出现的。
我发现了一个SO链接Get a Value from ActionResult in a ASP.Net Core API Method，它解释了如何响应reply/answer部分中的ActionResult<objecttype>返回值，其中单词“actual”是第一个定义的，您将在下面修改的代码中看到这个单词“actual”。
我修改过的代码贴在这里，代码部分内部和代码部分下方都有注解。代码内部的注解以“〈==”开头，后面是文本，直到“==〉”

// Initialize.
MOTRIP emptyMoTrip = new MOTRIP();
MOTRIP? resultMoTrip = new MOTRIP();
IEnumerable<MOTRIP> allTrips = Enumerable.Empty<MOTRIP>();
int daysPrevious = (int)(pTripType == eTripType.Any ? eDateRangeOffset.Week : eDateRangeOffset.Month);

// convert DateRangeOffset to 'dateonly' values.
DateOnly dtTo = DateOnly.FromDateTime( DateTime.Today);
DateOnly dtFrom = dtTo.AddDays(daysPrevious);

// Fetch the vehicle trips by date-range.
var result = await GetVehicleMOTripsByDateRange(UID_Vehicle.ToString(), dtFrom.ToString(), dtTo.ToString());
if ((result.Result as OkObjectResult) is null) { **<== this is the fix from the SO link.==>**
    return StatusCode(204, emptyMoTrip);  
}

var statusCode = (result.Result as OkObjectResult)!.StatusCode;
if (statusCode==204) {
    return StatusCode(204, emptyMoTrip);
}

**<== this next section allows code to get the result's DATA for further processing.==>**
var actual = (result.Result as OkObjectResult)!.Value as IEnumerable<MOTRIP>;
allTrips = (IEnumerable<MOTRIP>)actual!;

if ((allTrips is not null) && (!allTrips.Any())) {
    return StatusCode(204, emptyMoTrip);
}
<== this next section continues with business-logic related to the result-DATA.==>
if (allTrips is not null && allTrips.Any()) {
    switch (blah-blah-blah) {   
**<== the remainder of business logic is not shown as irrelevant to the "fix".==>**

请使用浏览器搜索“〈==”来查找我的代码注解。请使用浏览器搜索“actual”和“OkObjectResult”来查看相关的代码修复语句。