fix: ignore empty YAML documents in copier.yml

[sisp]

I've fixed loading copier.yml with empty YAML documents. The fix is simple: yaml.load_all() returns a sequence of YAML documents, and empty documents are represented by None, so I'm filtering None items using filter(None, yaml.load_all(...)). filter(None, <sequence>) actually filters any falsy items – not only Nones –, but that's no problem in this case.

Before, Copier raised an error

TypeError: 'NoneType' object is not subscriptable

which is not very helpful for debugging. Now, an empty YAML document is simply ignored. Of course, there's little sense in having empty YAML documents, but it's easy to have one unintentionally, e.g. when the file ends with the --- marker:

question:
  type: str
---
!include common.yml
---

In this regard, the YAML document syntax is a bit confusing IMO because a leading --- marker at the beginning of the file does not cause an empty YAML document. I think Copier should not be strict about it and ease template developers' lives. See also the confusion about this in #1496.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.30%. Comparing base (2f72105) to head (3f0675f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1538      +/-   ##
==========================================
+ Coverage   97.28%   97.30%   +0.02%     
==========================================
  Files          48       48              
  Lines        4564     4568       +4     
==========================================
+ Hits         4440     4445       +5     
+ Misses        124      123       -1     

Flag	Coverage Δ
unittests	97.30% <100.00%> (+0.02%)	⬆️

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[pawamoy]

Once again simple fix, makes lives easier, LGTM 🙂

[sisp]

Thanks for the quick review, @pawamoy! 🙏