fix: make sure transformer property on version endpoint is not undefi…

…ned (#523)

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# Description
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illustrations where appropriate. -->
The "transformer" property could be undefined in the event where
transformer is None, because the private property, _transformer is not
set in that case.

# Modifications
<!-- Summarize the key code changes. -->

# Tests
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- [ ] Deploying PyFunc models
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# Checklist
- [ ] Added PR label
- [ ] Added unit test, integration, and/or e2e tests
- [ ] Tested locally
- [ ] Updated documentation
- [ ] Update Swagger spec if the PR introduce API changes
- [ ] Regenerated Golang and Python client if the PR introduces API
changes

# Release Notes
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```release-note
Fix a bug in which transformer property is undefined for model version when there is no transformer found for the end point.
```

python/sdk/merlin/endpoint.py

@@ -82,6 +82,7 @@ def __init__(self, endpoint: client.VersionEndpoint, log_url: str = None):
                 )
 
             env_vars: Dict[str, str] = {}
+            self._transformer: Optional[Transformer] = None
             if transformer.env_vars is not None:
                 for env_var in transformer.env_vars:
                     if env_var.name is not None and env_var.value is not None:
@@ -97,7 +98,7 @@ def __init__(self, endpoint: client.VersionEndpoint, log_url: str = None):
                 resource_request=transformer_request, 
                 env_vars=env_vars,
             )
-            
+
         if log_url is not None:
             self._log_url = log_url
 
@@ -157,7 +158,7 @@ def resource_request(self) -> Optional[ResourceRequest]:
         return self._resource_request
 
     @property
-    def transformer(self) -> Transformer:
+    def transformer(self) -> Optional[Transformer]:
         return self._transformer
 
     @property