Switch usage to accept null

Author: Max Hniebergall

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/external/openai/OpenAiUnifiedStreamingProcessor.java

@@ -146,9 +146,10 @@ public static class ChatCompletionChunkParser {
             );
             PARSER.declareString(ConstructingObjectParser.constructorArg(), new ParseField(MODEL_FIELD));
             PARSER.declareString(ConstructingObjectParser.constructorArg(), new ParseField(OBJECT_FIELD));
-            PARSER.declareObject(
+            PARSER.declareObjectOrNull(
                 ConstructingObjectParser.optionalConstructorArg(),
                 (p, c) -> ChatCompletionChunkParser.UsageParser.parse(p),
+                null,
                 new ParseField(USAGE_FIELD)
             );
         }

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/external/openai/OpenAiUnifiedStreamingProcessorTests.java

@@ -131,11 +131,7 @@ public void testJsonLiteralCornerCases() {
                   ],
                   "model": "example_model",
                   "object": "chat.completion.chunk",
-                  "usage": {
-                    "completion_tokens": 50,
-                    "prompt_tokens": 20,
-                    "total_tokens": 70
-                  }
+                  "usage": null
                 }
             """;
         // Parse the JSON
@@ -150,10 +146,7 @@ public void testJsonLiteralCornerCases() {
             assertEquals("example_id", chunk.getId());
             assertEquals("example_model", chunk.getModel());
             assertEquals("chat.completion.chunk", chunk.getObject());
-            assertNotNull(chunk.getUsage());
-            assertEquals(50, chunk.getUsage().completionTokens());
-            assertEquals(20, chunk.getUsage().promptTokens());
-            assertEquals(70, chunk.getUsage().totalTokens());
+            assertNull(chunk.getUsage());
 
             List<StreamingUnifiedChatCompletionResults.ChatCompletionChunk.Choice> choices = chunk.getChoices();
             assertEquals(2, choices.size());