fix: don't generate license field if it's empty

[BLumia]

Describe the PR
Avoid generate license field with empty content in generated YAML and JSON files when running swag init

Relation issue
Intended to resolve #767

Additional context
Let me know if you need any additional info

[codecov]

Codecov Report

Merging #768 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #768      +/-   ##
==========================================
+ Coverage   82.10%   82.16%   +0.06%     
==========================================
  Files           8        8              
  Lines        1738     1744       +6     
==========================================
+ Hits         1427     1433       +6     
  Misses        193      193              
  Partials      118      118              

Impacted Files	Coverage Δ
parser.go	77.13% <100.00%> (+0.15%)	⬆️

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[sdghchj]

Thanks