OpenAiUsage throws `NullPointerException` in EmbeddingModel

[ThomasVitale]

Bug description
The methods implemented by OpenAiUsage throw NullPointerException whenever one of the usage type is not reported by OpenAI. For example, that's the case with the "generation tokens" for Embedding models.

Environment
Spring AI: 1.0.0-SNAPSHOT

Steps to reproduce

• Use the OpenAiEmbeddingModel to make a call to OpenAI
• On the EmbeddingResponse, get the Usage and call the getGenerationTokens() method.
• You'll get a NullPointerException.

Expected behavior
No NullPointerException when the usage tokens are not reported by the model provider.

[ThomasVitale]

Submitted a PR. @tzolov @sobychacko the OpenAI embedding model is currently broken without this fix