fix(adk): ensure streaming shell response is sent even when command produces no output

[zhuangjie1125]

…roduces no output

Fixed an edge case in ExecuteStreaming where no response was sent when the executed command produced no stdout output.

Main changes:

• Added hasOutput flag to track whether any output was received from stdout
• When scanner completes without any output, send an empty ExecuteResponse with default exit code (0) to ensure the stream properly terminates

Impact:

• Fixes potential client-side hanging when executing commands that produce no stdout (e.g., commands that only write to stderr or have no output at all)
• Ensures consistent streaming behavior regardless of command output

[github-actions]

Need to create a new tag

The following modules have changes and may need version updates:

• adk/backend/local (Current: adk/backend/local/v0.1.0)

⚠️ Please create and push new version tags for these modules after merging this PR.