fix(robot-server): Fix error parsing persisted loadModule commands (#…

…14509)

api/src/opentrons/protocol_engine/commands/load_module.py

@@ -87,7 +87,7 @@ class LoadModuleResult(BaseModel):
     )
 
     serialNumber: Optional[str] = Field(
-        ...,
+        None,
         description="Hardware serial number of the connected module. "
         "Will be `None` if a module is not electrically connected to the robot (like the Magnetic Block).",
     )