[Fix] Fixed MMDetWandbHook to operate on no-mask COCO

[MilkClouds]

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Motivation

MMDetWandbHook induce error when I used no-mask COCO dataset. (custom COCO dataset which I made)

Modification

I set default value of masks to [None], which is similar to COCO's no-mask mask [None, None, ...]. Since single None value of masks can mean masks is not well-formed, I discriminated usage of mask by checking masks[0] is not None

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos? If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

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Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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