Fix failing tests due to self_shielding attribute

Co-authored-by: Chris Havlin <[email protected]>

yt/frontends/ramses/data_structures.py

@@ -799,7 +799,7 @@ class RAMSESDataset(Dataset):
     force_cosmological: Optional[bool]
     _force_max_level: tuple[int, str]
     _bbox: Optional[list[list[float]]]
-    self_shielding: Optional[bool]
+    self_shielding: Optional[bool] = None
 
     def __init__(
         self,
@@ -1122,7 +1122,7 @@ def caster(val):
         has_namelist = self.read_namelist()
 
         if self.self_shielding is None and has_namelist:
-            nml = self.ds.parameters["namelist"]
+            nml = self.parameters["namelist"]
 
             # "self_shielding" is stored in physics_params in older versions of the code
             physics_params = nml.get("physics_params", default={})