bugfix: handle empty feature orders list (#571)

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# Description
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illustrations where appropriate. -->
The observation publisher is failing after the introduction of feature
orders field, because it was assumed that the feature orders is either
None or a list with one or more elements.

# Modifications
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# Tests
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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

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```release-note

```

python/observation-publisher/publisher/prediction_log_parser.py

@@ -27,7 +27,7 @@ def from_struct(
         :param inference_schema: Model inference schema.
         :return: Instance of PredictionLogFeatureTable.
         """
-        if inference_schema.feature_orders is not None:
+        if inference_schema.feature_orders is not None and len(inference_schema.feature_orders) > 0:
             columns = inference_schema.feature_orders
         else:
             assert isinstance(table_struct["columns"], ListValue)