Sample generate files improvements

[antgonza]

#2743 should be reviewed/merged first

[charles-cowart]

Request response to comments, before approving; changes not needed if you feel that it is fine.

[charles-cowart]

Is it better to return two empty sets vs None, or (None, None), throw an Exception, etc?

[charles-cowart]

Feels like it'd be more readable to test for not None or similar here, instead of having to go down into validate(), but I defer.

[codecov-io]

Codecov Report

Merging #2749 into dev will increase coverage by 4.68%.
The diff coverage is 93.33%.

@@            Coverage Diff            @@
##              dev   #2749      +/-   ##
=========================================
+ Coverage   89.62%   94.3%   +4.68%     
=========================================
  Files          93     166      +73     
  Lines        6459   19980   +13521     
=========================================
+ Hits         5789   18843   +13054     
- Misses        670    1137     +467

Impacted Files	Coverage Δ
qiita_db/metadata_template/prep_template.py	97.96% <100%> (ø)
qiita_db/metadata_template/sample_template.py	100% <100%> (ø)
...ita_db/metadata_template/base_metadata_template.py	95.51% <91.3%> (ø)
qiita_db/test/test_artifact.py	99.85% <0%> (ø)
qiita_db/metadata_template/__init__.py	100% <0%> (ø)
qiita_db/handlers/tests/test_prep_template.py	98.5% <0%> (ø)
qiita_db/test/test_logger.py	97.22% <0%> (ø)
qiita_db/handlers/tests/test_user.py	96.66% <0%> (ø)
qiita_db/test/test_software.py	99.82% <0%> (ø)
qiita_db/test/test_study.py	99.77% <0%> (ø)
... and 66 more

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