Fix an error of runtime with no bounding box of object detection

[tk26eng]

What this patch does to fix the issue.

This PR fix the error of runtime.
The problem occurs when no bounding box is detected with object detection task.
Then some values divided by zero, so the exception happens.

Link to any relevant issues or pull requests.

Fix #1162

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• Readability Approval for C++ from tkng, tsawada, primenumber

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[iizukak]

OA

[tk26eng]

@primenumber

Could you please review this PR ?

[primenumber]

RA for C++

[tk26eng]

/ready

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