Fix oss client create function (unitycatalog#1012)

**PR Checklist**

- [ ] A description of the changes is added to the description of this
PR.
- [ ] If there is a related issue, make sure it is linked to this PR.
- [ ] If you've fixed a bug or added code that should be tested, add
tests!
- [ ] If you've added or modified a feature, documentation in `docs` is
updated

**Description of changes**
get_function_async returns None if no function existing, fix the
corresponding error in create_function_async

<!-- Please state what you've changed and how it might affect the users.
-->

---------

Signed-off-by: serena-ruan <serena.rxy@gmail.com>

Author: serena-ruan

ai/core/src/unitycatalog/ai/core/client.py

@@ -464,14 +464,14 @@ async def create_function_async(
             )
 
         try:
-            await self.get_function_async(str(function_name), timeout=timeout)
-            if replace:
-                _logger.info(f"Function {function_name} already exists, replacing it.")
-                await self.delete_function_async(str(function_name), timeout=timeout)
-            else:
-                raise ValueError(
-                    f"Function {function_name} already exists. Set replace=True to overwrite it."
-                )
+            if await self.get_function_async(str(function_name), timeout=timeout):
+                if replace:
+                    _logger.info(f"Function {function_name} already exists, replacing it.")
+                    await self.delete_function_async(str(function_name), timeout=timeout)
+                else:
+                    raise ValueError(
+                        f"Function {function_name} already exists. Set replace=True to overwrite it."
+                    )
         except ServiceException:
             pass
 
@@ -739,7 +739,7 @@ def wrapper(x: int, y:int) -> int:
 
     async def get_function_async(
         self, function_name: str, timeout: Optional[float] = None, **kwargs: Any
-    ) -> FunctionInfo:
+    ) -> Optional[FunctionInfo]:
         """
         Retrieve a function by its full name asynchronously.
 

ai/core/tests/core/oss/test_oss_client.py

@@ -40,7 +40,7 @@
     FunctionsApi,
     SchemasApi,
 )
-from unitycatalog.client.exceptions import NotFoundException, ServiceException
+from unitycatalog.client.exceptions import NotFoundException
 
 SCHEMA = "uc_test"
 _logger = logging.getLogger(__name__)
@@ -763,8 +763,7 @@ async def test_create_function_invalid_data_type(uc_client):
 async def test_get_nonexistent_function(uc_client):
     function_name = f"{CATALOG}.{SCHEMA}.nonexistent_function"
 
-    with pytest.raises(ServiceException, match="(500)"):
-        uc_client.get_function(function_name=function_name)
+    assert uc_client.get_function(function_name=function_name) is None
 
 
 @pytest.mark.asyncio