Fix #935: DefaultAiService compatibility for TokenUsage result is null

langchain4j-core/src/main/java/dev/langchain4j/model/output/TokenUsage.java

@@ -85,13 +85,13 @@ public Integer totalTokenCount() {
  * <p>Fields which are null in both responses will be null in the result.
  *
  * @param that The token usage to add to this one.
- * @return a new {@link TokenUsage} instance with the token usage of both responses added together.
+ * @return a new {@link TokenUsage} instance with the token usage of both responses added together,
+ * or a new instance of the original fields if that token usage is null.
  */
  public TokenUsage add(TokenUsage that) {
  if (that == null) {
  return new TokenUsage(inputTokenCount, outputTokenCount, totalTokenCount);
  }
-
  return new TokenUsage(
  sum(this.inputTokenCount, that.inputTokenCount),
  sum(this.outputTokenCount, that.outputTokenCount),

langchain4j-core/src/test/java/dev/langchain4j/model/output/TokenUsageTest.java

@@ -89,7 +89,10 @@ public void test_add() {
  new TokenUsage(null, null, null)
  .add(new TokenUsage(null, null, null)))
  .isEqualTo(new TokenUsage(null, null, null));
+ }
 
+ @Test
+ public void test_add_null() {
  assertThat(new TokenUsage(1, 2, 3).add(null))
  .isEqualTo(new TokenUsage(1, 2, 3));
  }

langchain4j-local-ai/src/main/java/dev/langchain4j/model/localai/LocalAiChatModel.java

@@ -104,7 +104,7 @@ private Response<AiMessage> generate(List<ChatMessage> messages,
 
  return Response.from(
  aiMessageFrom(response),
- null,
+ tokenUsageFrom(response.usage()),
  finishReasonFrom(response.choices().get(0).finishReason())
  );
  }

langchain4j-open-ai/src/main/java/dev/langchain4j/model/openai/InternalOpenAiHelper.java

@@ -237,7 +237,7 @@ private static ToolExecutionRequest toToolExecutionRequest(ToolCall toolCall) {
 
  public static TokenUsage tokenUsageFrom(Usage openAiUsage) {
  if (openAiUsage == null) {
- return null;
+ return new TokenUsage();
  }
  return new TokenUsage(
  openAiUsage.promptTokens(),