Disallow null values in input YAML

[apragsdale]

Closes #340

[codecov]

Codecov Report

Merging #387 (08fd822) into main (fb694c3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 08fd822 differs from pull request most recent head 7d39cff. Consider uploading reports for the commit 7d39cff to get more accurate results

@@           Coverage Diff           @@
##             main     #387   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
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  Files           5        5           
  Lines        1529     1536    +7     
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+ Hits         1527     1534    +7     
  Misses          2        2           

Impacted Files	Coverage Δ
demes/load_dump.py	100.00% <100.00%> (ø)
demes/demes.py	100.00% <0.00%> (ø)

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[apragsdale]

Essentially what I've done here is add a function to load_dump.py that iterates through the loaded data dict and checks for any None values, raising an error if it finds one. There may be a much more elegant way of doing this, and this solution feels a bit hacky. It does seem to work though, and if we're just forbidding null entries in the YAML until we make a final decision on what they mean and how they should be handled, this could be a reasonable stopgap.

Thoughts @jeromekelleher @grahamgower @molpopgen?

[grahamgower]

LGTM. I can't think of a cleaner way to do this, and your approach is at least simple and self contained. Can you update the commit message to indicate the change before you merge this?