Allow 'None' in 'dose_per_frame' key in 'Base' sub-class

Author: tieneupin

src/murfey/util/models.py

@@ -368,7 +368,7 @@ class PreprocessingParametersTomo(BaseModel):
     eer_fractionation: int
 
     class Base(BaseModel):
-        dose_per_frame: float
+        dose_per_frame: Optional[float]  # Doesn't match the model?
         gain_ref: Optional[str]
         manual_tilt_offset: float
         eer_fractionation: int