Do not support None context

Author: dandavison

temporalio/converter.py

@@ -139,7 +139,7 @@ class WithSerializationContext(ABC):
     during serialization and deserialization.
     """
 
-    def with_context(self, context: Optional[SerializationContext]) -> Self:
+    def with_context(self, context: SerializationContext) -> Self:
         """Return a copy of this object configured to use the given context.
 
         Args: