Handle null/empty values better for training and consumption

**System Information (please complete the following information):**
- Model Builder Version: ml.net CLI 16.1.1
- Visual Studio Version 8.7.6 for macs

**Describe the bug**
Tried to change model input to support nullable types to support my data set, ConsumeModel throws error:

"System.ArgumentOutOfRangeException has been thrown, "Could not determine an IDataView type for member Flash_point (Parameter 'rawType')""

Have inputs, but some are missing values. (Empty cell, but are NOT zero)

**To Reproduce**
Steps to reproduce the behavior:
1. Train model with data with missing values
2. Try to run new data with missing values thru model
3. Change modelinput float options to float?
4. See error

**Expected behavior**
Handle nullable types since my data includes some empty cells

**Screenshots**
NA

**Additional context**
NA