Reverted back to invalid None assignment

Update methods should really take partial models with all-optional
contents (SciCatProject#58). Rather than fixing that issue, let's just ignore
the typing error for now.

pyscicat/client.py

@@ -489,7 +489,8 @@ def proposals_update(
         if proposalId is None:
             assert proposal.proposalId is not None, "proposalId should not be None"
             proposalId = proposal.proposalId
-        proposal.proposalId = ""
+        # TODO updates should allow partial proposals, where all fields are optional. See #58
+        proposal.proposalId = None  # type: ignore [assignment]
         result = self._call_endpoint(
             cmd="patch",
             endpoint=f"Proposals/{quote_plus(proposalId)}",