[Enhance] Ensure metrics is not empty when saving best ckpts

[zhouzaida]

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Motivation

Resolve #845

During initial training, metrics produced by the poor model may be an empty dict and it will cause errors when saving the best checkpoints by CheckpointHook. Therefore, this PR ensures metrics are not empty before saving the best checkpoints.

Modification

Add checking for metrics in CheckpointHook.after_val_epoch method.

BC-breaking (Optional)

No

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

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.

[codecov]

Codecov Report

Base: 78.66% // Head: 78.24% // Decreases project coverage by -0.42% ⚠️

Coverage data is based on head (1160aac) compared to base (fe26c65).
Patch coverage: 46.00% of modified lines in pull request are covered.

❗ Current head 1160aac differs from pull request most recent head d94a5a8. Consider uploading reports for the commit d94a5a8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #849      +/-   ##
==========================================
- Coverage   78.66%   78.24%   -0.43%     
==========================================
  Files         128      130       +2     
  Lines        9348     9524     +176     
  Branches     1848     1887      +39     
==========================================
+ Hits         7354     7452      +98     
- Misses       1679     1752      +73     
- Partials      315      320       +5     

Flag	Coverage Δ
unittests	78.24% <46.00%> (-0.43%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmengine/model/wrappers/__init__.py	66.66% <ø> (-3.34%)	⬇️
mmengine/optim/optimizer/zero_optimizer.py	52.17% <0.00%> (-4.97%)	⬇️
mmengine/registry/build_functions.py	80.20% <ø> (ø)
mmengine/registry/default_scope.py	100.00% <ø> (ø)
mmengine/runner/runner.py	84.96% <ø> (-0.19%)	⬇️
mmengine/structures/base_data_element.py	93.65% <ø> (ø)
mmengine/structures/label_data.py	100.00% <ø> (ø)
mmengine/visualization/vis_backend.py	84.49% <ø> (ø)
mmengine/visualization/visualizer.py	93.19% <ø> (ø)
mmengine/runner/checkpoint.py	52.07% <14.28%> (+8.11%)	⬆️
... and 21 more

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