minor fix in htc tta when num det boxes is 0

[thangvubk]

No description provided.

[codecov]

Codecov Report

Merging #4516 (48258d9) into master (3a9f5ef) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4516      +/-   ##
==========================================
- Coverage   62.09%   61.98%   -0.11%     
==========================================
  Files         241      241              
  Lines       18400    18400              
  Branches     3085     3085              
==========================================
- Hits        11426    11406      -20     
- Misses       6416     6439      +23     
+ Partials      558      555       -3     

Flag	Coverage Δ
unittests	61.95% <ø> (-0.10%)	⬇️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/htc_roi_head.py	31.83% <ø> (ø)
mmdet/models/roi_heads/test_mixins.py	45.03% <0.00%> (-8.40%)	⬇️
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	74.45% <0.00%> (-1.91%)	⬇️

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