fix(TokenUsage): Token can't be null in AI Response (langchain4j#1295)

## Issue
Fixes langchain4j#1294

## Change
Allow TokenUsage to be null in the Response. 


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## Checklist for adding new model integration
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- [ ] I have added my new module in the
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## Checklist for adding new embedding store integration
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## Checklist for changing existing embedding store integration
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`{NameOfIntegration}EmbeddingStore` works correctly with the data
persisted using the latest released version of LangChain4j

langchain4j/src/main/java/dev/langchain4j/service/Result.java

@@ -28,7 +28,7 @@ public class Result<T> {
     @Builder
     public Result(T content, TokenUsage tokenUsage, List<Content> sources, FinishReason finishReason) {
         this.content = ensureNotNull(content, "content");
-        this.tokenUsage = ensureNotNull(tokenUsage, "tokenUsage");
+        this.tokenUsage = tokenUsage;
         this.sources = copyIfNotNull(sources);
         this.finishReason = finishReason;
     }