Guard remote config response parsing from empty FlatBuffer

[triplef]

Description

Guard RemoteConfigResponse::MarkCompleted() from attempting to read a FlatBuffer when the builder holds no data, preventing the desktop Remote Config client from crashing on empty responses.

Fixes #1433.

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Testing

Not run (Remote Config desktop integration tests depend on internal credentials; change is limited to an early-exit guard).

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Type of Change

• Bug fix. Add the issue # below if applicable. — [Bug] Crash in flatbuffers::GetMutableRoot<T> #1433
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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[gemini-code-assist]

Summary of Changes

Hello @triplef, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a critical safeguard to the Remote Config client, specifically within the MarkCompleted() method. By adding a check for empty FlatBuffer data, it prevents potential crashes that would occur if the system attempted to process a response without any content. This ensures the stability and robustness of the desktop Remote Config client when handling various server responses.

Highlights

• Bug Fix: Addressed an issue where the desktop Remote Config client could crash when attempting to parse an empty FlatBuffer response.
• Defensive Programming: Implemented a guard condition in RemoteConfigResponse::MarkCompleted() to prevent FlatBuffer parsing if the builder holds no data or has a null buffer pointer.

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[gemini-code-assist]

Code Review

This pull request aims to fix a crash when parsing an empty remote config response by adding a guard before accessing the FlatBuffer. While the intention is correct, the added check for an empty buffer (GetSize() == 0) is not sufficient to prevent all out-of-bounds access. A malformed response with a very small (but non-zero) body could still cause a crash. I've provided a suggestion to make this check more robust by ensuring the buffer is large enough to contain a root pointer.