Use missing instead of nothing in `DataFrame()` overload

[gabrieldansereau]

What the pull request does

As mentioned in #52, this replaces nothing elements by missing when converting a layer to a DataFrame, as missing has better support.

Type of change

Please indicate the relevant option(s)

• [ ] :bug: Bug fix (non-breaking change which fixes an issue)
• [ ] :sparkle: New feature (non-breaking change which adds functionality)
• [x] :boom: Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] :book: This change requires a documentation update

Checklist

• [x] The changes are documented
  • [x] The docstrings of the different functions describe the arguments, purpose, and behavior
  • [ ] There are examples in the documentation website
• [x] The changes are tested
• [x] The changes do not modify the behavior of the previously existing functions
  • If they do, please explain why and how in the introduction paragraph
• [x] For new contributors - my name and information are added to .zenodo.json
• [x] The Project.toml field version has been updated
  • Change the last number for a v0.0.x series release, a bug fix, or a performance improvement
  • Change the middle number for additional features that do not break the current test suite (unless you fix a bug in the test suite)
  • Change the first number for changes that break the current test suite

[codecov-commenter]

Codecov Report

Merging #105 (89beeaf) into master (f27ea63) will increase coverage by 0.21%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
+ Coverage   80.02%   80.23%   +0.21%     
==========================================
  Files          24       24              
  Lines         756      759       +3     
==========================================
+ Hits          605      609       +4     
+ Misses        151      150       -1     

Flag	Coverage Δ
unittests	80.23% <100.00%> (+0.21%)	:arrow_up:

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Impacted Files	Coverage Δ
src/SimpleSDMLayers.jl	92.30% <ø> (ø)
src/integrations/DataFrames.jl	100.00% <100.00%> (+1.51%)	:arrow_up:

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