Fix Assistant exception with OpenAI #1017
                
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OpenAI can report tool_calls: null. Which causes OpenAI assistant to raise exceptions due to the following check:
raise ArgumentError, "Tool calls must be an array of hashes" unless tool_calls.is_a?(Array) && tool_calls.all? { |tool_call| tool_call.is_a?(Hash) }
This fix is to make the check consider nil as an empty array. This change is related to #1011