[Fix] Fix solov2 cannot dealing with empty gt image

[BIGWangYuDong]

Solve PR: #9159

loss_mask may empty when num_pos = 0. Hence, fix the num_pos=0 case to support dealing with empty gt image

[Czm369]

LGTM

[codecov]

Codecov Report

Base: 64.21% // Head: 64.18% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (d61c060) compared to base (5d06246).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #9185      +/-   ##
==========================================
- Coverage   64.21%   64.18%   -0.03%     
==========================================
  Files         361      361              
  Lines       29567    29567              
  Branches     5029     5029              
==========================================
- Hits        18985    18978       -7     
- Misses       9573     9583      +10     
+ Partials     1009     1006       -3     

Flag	Coverage Δ
unittests	64.16% <0.00%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/models/dense_heads/solov2_head.py	10.13% <0.00%> (ø)
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	68.33% <0.00%> (-1.39%)	⬇️

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