Onnx with single float output throws exception during MLContext predict

**System Information (please complete the following information):**
- OS & Version: [Windows 11]
- ML.NET Version: [ML.NET v3.0.1]
- .NET Version: [.NET 8.0]
- Microsoft.ML.OnnxRuntime Version: [1.1.9.1]
- Microsoft.ML.OnnxTransformer Version: [1.1.9.1]
-
**Describe the bug**
I have an onnx file with only a single float32 output. By using the following class to map the output with mlContext.Transforms.ApplyOnnxModel, and CreatePredictionEngine, predict function will report System.ArgumentNullException: Value cannot be null. (Parameter 'source') at Microsoft.ML.Transforms.Onnx.OnnxTransformer.Mapper.<>c__DisplayClass16_0`1.

I can use InferenceSession to run it without problem.

**To Reproduce**
Steps to reproduce the behavior:
1. Clone https://github.com/xqiu/ml_onnx_output_error
2. Run the program to see the bug appearance

Code is:

~~~
public class ModelInput
{
[ColumnName("input")]
[VectorType(1, 1, 40, 40)] // Adjust dimensions to match your model input
public float[] Features { get; set; } = new float[1 * 1 * 40 * 40]; // Flattened 4D array for ONNX input
}

public class ModelOutput
{
[ColumnName("output")]
public float Prediction { get; set; }
}

public static void PredictWithMLContextSingle(string modelPath)
{
Console.WriteLine("\r\nRun PredictWithMLContext:");

var mlContext = new MLContext();

// Define the ONNX pipeline
var pipeline = mlContext.Transforms.ApplyOnnxModel(
modelFile: modelPath,
outputColumnNames: new[] { "output" },
inputColumnNames: new[] { "input" });
// Prepare input data (replace this with your actual input data)
var input = new ModelInput();
for (int i = 0; i < input.Features.Length; i++)
{
input.Features[i] = 0.1f; // Example value, replace with your actual data
}

// Create the data view with a single item wrapped in a list
var emptyDataView = mlContext.Data.LoadFromEnumerable(new[] { input });

// Fit the pipeline
var mlModel = pipeline.Fit(emptyDataView);

// Load the model
var predictionEngine = mlContext.Model.CreatePredictionEngine(mlModel);

// Perform prediction
var output = predictionEngine.Predict(input);

Console.WriteLine("Using MLContext Prediction result:");
Console.WriteLine($"Output: {output}");
}

~~~

**Expected behavior**
Prediction works with MLContext.

**Screenshots, Code, Sample Projects**
Sample code in https://github.com/xqiu/ml_onnx_output_error

**Additional context**
Add any other context about the problem here.