[BUG] [RPC] fix BE crash in SendRpcResponse when high concurrency

[xinyiZzz]

Proposed changes

Close related #7412 (replace it with issue number if it exists).

The response is accessed when done->Run is called in transmit_data(),
give response a default value to avoid null pointers in high concurrency.

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• I have created an issue on (Fix [Bug] When high concurrency, BE crash reports MissingRequiredFields in SendRpcResponse #7412) and described the bug/feature there in detail
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[yangzhg]

LGTM

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PR approved by at least one committer and no changes requested.

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PR approved by anyone and no changes requested.