"Start Previewing Node" Does Not Work For Post Process Materials

Using "start previewing node" in a post process material does not work. The preview will only show the final result that is connected to the material attributes. 

Working as expected in 4.18 CL# 3832480

Found in 4.19 CL# 4033788, 4.20 CL# 4369336, 4.21 CL# 4408769

This is a regression.

1. Open the editor
2. Create a new material
3. Set its Domain to Post Process
4. Place a "Constant 3Vector" node and set R to 1
5. Right click the node and select "Start Previewing Node"

Results:
The preview will stay black

Expected:
The preview would show red like the "Constant 3Vector" is set

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