fix: make assetsummary computable for dandisets with no assets

[satra]

allows computing assetsummary for dandisets with no assets
changes assetmodel to allow for numberoffiles and bytes to be 0 till publish time.

[codecov]

Codecov Report

Merging #38 (a098b99) into master (b38ce14) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   95.75%   95.79%   +0.04%     
==========================================
  Files          11       11              
  Lines         965      975      +10     
==========================================
+ Hits          924      934      +10     
  Misses         41       41              

Flag	Coverage Δ
unittests	95.79% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandischema/consts.py	100.00% <100.00%> (ø)
dandischema/metadata.py	97.09% <100.00%> (+0.01%)	⬆️
dandischema/models.py	93.81% <100.00%> (+0.07%)	⬆️
dandischema/tests/test_metadata.py	100.00% <100.00%> (ø)
dandischema/tests/test_models.py	94.73% <100.00%> (+0.09%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b1e9ec3...a098b99. Read the comment docs.