Fix the following NPE:
java.lang.NullPointerException: Cannot invoke "java.lang.Integer.longValue()" because the return value of "org.springframework.ai.openai.api.OpenAiApi$Usage.completionTokens()" is null at org.springframework.ai.openai.metadata.OpenAiUsage.getGenerationTokens(OpenAiUsage.java:55) ~[spring-ai-openai-1.0.0-20240802.202856-438.jar:1.0.0-SNAPSHOT] at org.springframework.ai.model.observation.ModelUsageMetricsGenerator.generate(ModelUsageMetricsGenerator.java:52) ~[spring-ai-core-1.0.0-20240802.202856-485.jar:1.0.0-SNAPSHOT] at org.springframework.ai.embedding.observation.EmbeddingModelMeterObservationHandler.onStop(EmbeddingModelMeterObservationHandler.java:41) ~[spring-ai-core-1.0.0-20240802.202856-485.jar:1.0.0-SNAPSHOT] at org.springframework.ai.embedding.observation.EmbeddingModelMeterObservationHandler.onStop(EmbeddingModelMeterObservationHandler.java:29) ~[spring-ai-core-1.0.0-20240802.202856-485.jar:1.0.0-SNAPSHOT] at io.micrometer.observation.SimpleObservation.lambda$notifyOnObservationStopped$0(SimpleObservation.java:268) ~[micrometer-observation-1.13.0.jar:1.13.0] at java.base/java.util.ArrayDeque$DescendingIterator.forEachRemaining(ArrayDeque.java:772) ~[na:na] at io.micrometer.observation.SimpleObservation.notifyOnObservationStopped(SimpleObservation.java:268) ~[micrometer-observation-1.13.0.jar:1.13.0] at io.micrometer.observation.SimpleObservation.stop(SimpleObservation.java:188) ~[micrometer-observation-1.13.0.jar:1.13.0] at io.micrometer.observation.Observation.observe(Observation.java:572) ~[micrometer-observation-1.13.0.jar:1.13.0] at org.springframework.ai.openai.OpenAiEmbeddingModel.call(OpenAiEmbeddingModel.java:161) ~[spring-ai-openai-1.0.0-20240802.202856-438.jar:1.0.0-SNAPSHOT] at org.springframework.ai.embedding.EmbeddingModel.embed(EmbeddingModel.java:56) ~[spring-ai-core-1.0.0-20240802.202856-485.jar:1.0.0-SNAPSHOT] at org.springframework.ai.embedding.EmbeddingModel.embed(EmbeddingModel.java:39) ~[spring-ai-core-1.0.0-20240802.202856-485.jar:1.0.0-SNAPSHOT]