ConvertBoxToStandardModed cannot handle empty boxes

[Can-Zhao]

Describe the bug
When input boxes are empty [], it got error since the boxes are supposed to have shape [N,4] or [N,6].

ValueError: Currently we support only boxes with shape [N,4] or [N,6], got boxes with shape torch.Size([1, 0])

To Reproduce
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Expected behavior
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Environment

Ensuring you use the relevant python executable, please paste the output of:

python -c 'import monai; monai.config.print_debug_info()'

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