Fix model provider of vertex ai

[kazukitakamatsu]

Summary

This PR fixed the initialization process when the service account key is omitted in the model provider of Google Cloud Vertex AI.
The service account key was originally optional, but an error occurred when trying to json decode the string from the omitted case.

Screenshots

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods