[Bug Report]RegistryManager.GetDeviceAsync() returns null

# Context

- **OS, version, SKU and CPU architecture used:** Azure Function Dotnet isolated Windows
- **Application's .NET Target Framework :** net8.0
- **Device:** N/A
- **SDK version used:**

## Description of the issue
RegistryManager.GetDeviceAsync (string deviceId) returns a null if device is not found.
Per Code Expected DeviceNotFoundException

``` public virtual Task GetDeviceAsync(string deviceId, CancellationToken cancellationToken)
{
if (Logging.IsEnabled)
Logging.Enter(this, $"Getting device: {deviceId}", nameof(GetDeviceAsync));
try
{
if (string.IsNullOrWhiteSpace(deviceId))
{
throw new ArgumentException(IotHubApiResources.GetString(ApiResources.ParameterCannotBeNullOrWhitespace, "deviceId"));
}

EnsureInstanceNotClosed();
var errorMappingOverrides = new Dictionary>>()
{
{ HttpStatusCode.NotFound, async responseMessage => new DeviceNotFoundException(await ExceptionHandlingHelper.GetExceptionMessageAsync(responseMessage).ConfigureAwait(false)) }
};

return _httpClientHelper.GetAsync(GetRequestUri(deviceId), errorMappingOverrides, null, false, cancellationToken);
```
## Code sample exhibiting the issue
[Fact]
public async Task TestRegistryManager()
{
var registry = RegistryManager.CreateFromConnectionString(connectionString);

await Assert.ThrowsAsync(async () => await registry.GetDeviceAsync("IDoNoTExist") );
}

## Console log of the issue
Follow the instructions [here](https://github.com/Azure/azure-iot-sdk-csharp/tree/main/tools/CaptureLogs) to capture SDK logs.
Don't forget to remove any connection string information!

Start at RegistryManager.GetDeviceAsync(string deviceId) and compare its not-found handling with the behavior shown in TestRegistryManager. Reproduce the call with the non-existent device ID "IDoNoTExist" and verify whether the result is null or a DeviceNotFoundException. Done means the behavior matches the documented exception expectation and the test passes.

Written by the indexing model from the issue text.