Allow None for phys prop on joint

Author: domfournier

simpeg_drivers/joint/options.py

@@ -44,7 +44,7 @@ class BaseJointOptions(BaseData):
     run_command: str = "simpeg_drivers.driver"
     conda_environment: str = "simpeg-drivers"
     forward_only: bool = False
-    physical_property: str = ""
+    physical_property: str | None = None
 
     group_a: SimPEGGroup
     group_a_multiplier: float = 1.0