Handle JSON null string while using structured output validation

[nicolaskrier]

Description

I noticed that structured output validation fails when a model emits "null" as its JSON output. These changes handle a null JSON node as a passed validation instead of triggering unnecessary retries.

Reproduction

I performed some successful tests of these changes with my tools example by using Ollama and Mistral AI model providers. As a first step, I applied #6974 fix. You can find below screenshots of what I observed on Phoenix.

Behavior of my tools example before the fix with Ollama

Behavior of my tools example after the fix with Ollama

Behavior of my tools example before the fix with Mistral AI

Behavior of my tools example after the fix with Mistral AI

Testing

I added a unit test in StructuredOutputValidationAdvisorTests class.

Notes

• It could be worth testing with other model providers.
• With Mistral AI, the model is invoked multiple times to retrieve tool calls to perform. This appears to be a separate issue worth investigating. I haven't yet identified why this occurs.
• The model may not follow the system prompt instructing it to return "null" if the tool call response is null. In that case, it returns a JSON response.

[nicolaskrier]

I figured out that I had to modify the system prompt to have mistral-medium-latest behaving as intended. Now it works fine for Ollama and Mistral AI model providers. Nevertheless, I think having to define this kind of system prompt is cumbersome. I wonder if something could be done on Spring AI side to ease this behavior.