fix: Treat empty nodes as zero values

[lukemassa]

Before submitting your PR, please confirm the following.

• Describe the purpose for which you created this PR.
• Create test code that corresponds to the modification

This code begins to address #414. Basically the idea is, "empty sequence entry" is not actually an "error" and should be treated as a "NullNode" instead.

There are still several additional issues code doesn't yet address, like:

• foo:\n- is treated like a zero length list (which I think is the same behavior as a bug in go-yaml v3 skips blank elements in a sequence whereas v2 does not go-yaml/yaml#1011)
• foo:\n - errors out with "string was used where sequence is expected" (when it also should be treated as null)

Looking for confirmation that I've identified an actual issue and am on the right track to fix it, at which point I will continue trying to address and test for these additional cases.

[codecov-commenter]

Codecov Report

Merging #415 (28df8de) into master (0640a15) will decrease coverage by 0.06%.
The diff coverage is 78.57%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
- Coverage   76.02%   75.96%   -0.06%     
==========================================
  Files          13       13              
  Lines        4692     4694       +2     
==========================================
- Hits         3567     3566       -1     
- Misses        866      867       +1     
- Partials      259      261       +2