Allow NAs in missing columns

[dshemetov]

Part of fixing cmu-delphi/covidcast-indicators#1520. This should be merged first, as it will be a no-op (aside from a memory usage increase on the acquisition server). Then we can merge the archivediffer fix (upcoming).

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

Summary

Allows NAs in missing columns, so the validation can fix them. This will allow archive differ to easily add deletion codes, not have to do any validation on the rest of the missing codes, and let the validation here in acquisition take care of it.

[krivard]

Nice work!