fix: default value of google storage sa to empty

[kazukitakamatsu]

Summary

Fixed an error when using Default Service Account in Google Storage.
When using Default Service Account, the environment variable GOOGLE_STORAGE_SERVICE_ACCOUNT_JSON_BASE64 must be an empty string. However, even if an empty string was specified in .env, the default string was specified in docker-compose.yaml. Therefore, the default string was changed to the empty string.

Additional Information.

As an alternative fix, it is possible to change the default to be specified only when undefined, but since the scope of impact is large, this was addressed in this issue.

current style
SOME_VARIABLE: ${SOME_VARIABLE:-default}
alternative style
SOME_VARIABLE: ${SOME_VARIABLE-default}

see https://docs.docker.com/reference/compose-file/interpolation/

Screenshots

Checklist

• 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

[crazywoola]

LGTM