Fix TypeError in AssistantMessageNormalizer when toolCalls is null

[Muneer-Shafi]

Solution

• Use $data->hasToolCalls() to check for tool calls existence before mapping
• Return $data->content when no tool calls are present
• Maintains proper return type annotation for both scenarios

Changes

• Modified AssistantMessageNormalizer::normalize() method
• Added proper null safety check
• Preserves existing functionality for messages with tool calls

Testing

• Tested with AssistantMessage containing tool calls
• Tested with AssistantMessage without tool calls (null toolCalls)
• No breaking changes to existing API

Closes #[302]

[github-actions]

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