get_papers will fail if one of the paper ids isn't found

[JessFairbairn]

• I'm submitting a ...
  [X ] bug report
  [ ] feature request
  [ ] question about the decisions made in the repository
  [ ] question about how to use this project

• Summary
  When one of the paper IDs sent to the bulk paper lookup endpoint isn't found, the API returns a null in that position. This then fails in the constructor for the Paper class (specifically in the _initAttributes method).

• Other information (e.g. detailed explanation, stack traces, related issues, suggestions how to fix, links for us to have context, eg. StackOverflow, personal fork, etc.)
  Think this can probably be fixed with a simple null check in the .map function