[Enhancement]: Support empty val.

[RangiLyu]

When dataset is too small, the detection results at the val dataset may be empty at the first val phase.
Now I add a warning instead of rasing an error since many people want this feature.

[codecov]

Codecov Report

Merging #310 (487b3c7) into main (bd1fa26) will decrease coverage by 0.10%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
- Coverage   73.27%   73.17%   -0.11%     
==========================================
  Files          60       60              
  Lines        3937     3944       +7     
  Branches      647      649       +2     
==========================================
+ Hits         2885     2886       +1     
- Misses        859      864       +5     
- Partials      193      194       +1     

Flag	Coverage Δ
unittests	73.17% <14.28%> (-0.11%)	⬇️

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Impacted Files	Coverage Δ
nanodet/evaluator/coco_detection.py	86.04% <14.28%> (-13.96%)	⬇️

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