change default to None for forces and stress to prevent Pydantic vali…

…dation errors on older calculations that did not calculate those fields

emmet-core/emmet/core/vasp/task_valid.py

@@ -68,10 +68,10 @@ class OutputSummary(BaseModel):
  None, description="The DFT bandgap for the last calculation"
  )
  forces: Optional[List[Vector3D]] = Field(
- [], description="Forces on atoms from the last calculation"
+ None, description="Forces on atoms from the last calculation"
  )
  stress: Optional[Matrix3D] = Field(
- [], description="Stress on the unitcell from the last calculation"
+ None, description="Stress on the unitcell from the last calculation"
  )