Fix type issue with widgets.getArgument (#581)

## Changes
<!-- Summary of your changes that are easy to understand -->
- The `str | None` used previously doesn't work with older version of
python, fixing it so that it unblocks integration test and release

## Tests
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How is this tested? Please see the checklist below and also describe any
other relevant tests
-->

- [x] `make test` run locally
- [x] `make fmt` applied
- [x] relevant integration tests applied

Author: edwardfeng-db

databricks/sdk/runtime/dbutils_stub.py

@@ -288,7 +288,7 @@ def get(name: str) -> str:
             ...
 
         @staticmethod
-        def getArgument(name: str, defaultValue: typing.Optional[str] = None) -> str | None:
+        def getArgument(name: str, defaultValue: typing.Optional[str] = None) -> typing.Optional[str]:
             """Returns the current value of a widget with give name.
             :param name: Name of the argument to be accessed
             :param defaultValue: (Deprecated) default value