Fixing Deserialization Failure for Enum with Empty String finish_reason

Start at org.springframework.ai.openai.api.OpenAiApi$ChatCompletionFinishReason and reproduce the reported ChatCompletionChunk JSON with an empty finish_reason. Confirm that the enum deserializes both an empty string and null without the InvalidFormatException; the issue is done when the provided payload can be parsed normally.

Bug description
Deserialization of org.springframework.ai.openai.api.OpenAiApi$ChatCompletionFinishReason fails when the finish_reason field is an empty string "" rather than null during the step-by-step inference process. For instance, while the model is inferring step-by-step, the finish_reason field typically returns an empty string "" or null until all tokens are returned. Due to the following enum definition, "" cannot be directly handled:
public static enum ChatCompletionFinishReason {
@JsonProperty("stop")
STOP,
@JsonProperty("length")
LENGTH,
@JsonProperty("content_filter")
CONTENT_FILTER,
@JsonProperty("tool_calls")
TOOL_CALLS,
@JsonProperty("tool_call")
TOOL_CALL;

    private ChatCompletionFinishReason() {
    }
}

Environment
Please provide as many details as possible: Spring AI version, Java version, which vector store you use if any, etc
1.0.0-M6
Steps to reproduce
Steps to reproduce the issue.

Expected behavior
A clear and concise description of what you expected to happen.
when finish_reason is "",it can be deserialized normally

Minimal Complete Reproducible example
Please provide a failing test or a minimal complete verifiable example that reproduces the issue.
Bug reports that are reproducible will take priority in resolution over reports that are not reproducible.
com.fasterxml.jackson.databind.exc.InvalidFormatException: Cannot coerce empty String ("") to org.springframework.ai.openai.api.OpenAiApi$ChatCompletionFinishReason value (but could if coercion was enabled using CoercionConfig)
at [Source: (String)"{"id":"xxxxxx","object":"chat.completion.chunk","created":1742218321,"model":"xxxx","choices":[{"index":0,"delta":{"role":"assistant","content":"","tool_calls":null,"reasoning_content":null},"logprobs":null,"finish_reason":"","matched_stop":null}],"usage":null}"; line: 1, column: 257] (through reference chain: org.springframework.ai.openai.api.OpenAiApi$ChatCompletionChunk["choices"]->java.util.ArrayList[0]->org.springframework.ai.openai.api.OpenAiApi$ChatCompletionChunk$ChunkChoice["finish_reason"])