extractors/bilibili: handle the situation that the season_type is empty

[iawia002]

fix #365

[codecov]

Codecov Report

Merging #366 into master will decrease coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage   71.83%   71.68%   -0.15%     
==========================================
  Files          29       29              
  Lines        2084     2084              
==========================================
- Hits         1497     1494       -3     
- Misses        424      428       +4     
+ Partials      163      162       -1

Impacted Files	Coverage Δ
extractors/bilibili/bilibili.go	76.92% <100%> (+1.02%)	⬆️
extractors/qq/qq.go	77.65% <0%> (-7.45%)	⬇️
extractors/iqiyi/iqiyi.go	87.38% <0%> (+1.8%)	⬆️

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