Fix #935: DefaultAiService compatibility for TokenUsage result is null

[Kugaaa]

Context

See #935
LocalAiChatModel.generate the response of generate returned that tokenUsage is null

Change

• LocalAiChatModel tried to return TokenUsage result
• TokenUsage.add return if origin token usage is null, ensure AiService proxy successfully executed

Checklist

Before submitting this PR, please check the following points:

• I have added unit and integration tests for my change
• All unit and integration tests in the module I have added/changed are green
• All unit and integration tests in the core and main modules are green
• I have added/updated the documentation
• I have added an example in the examples repo (only for "big" features)
• I have added my new module in the BOM (only when a new module is added)

Checklist for adding new embedding store integration

• I have added a {NameOfIntegration}EmbeddingStoreIT that extends from either EmbeddingStoreIT or EmbeddingStoreWithFilteringIT

[langchain4j]

@Kugaaa thank you, but this has been fixed already by another PR: #939