Fix: when alias returns a falsy value, it may return undefined

[jimmycallin]

Hi! I had an aliased field vanishing from the local state, which after some investigation seem to have been solved by splitting up the return statement as shown in this PR.

[apollo-cla]

@jimmycallin: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[codecov-io]

Codecov Report

Merging #303 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #303      +/-   ##
==========================================
+ Coverage      95%   95.12%   +0.12%     
==========================================
  Files           2        2              
  Lines          80       82       +2     
  Branches       26       26              
==========================================
+ Hits           76       78       +2     
  Misses          1        1              
  Partials        3        3

Impacted Files	Coverage Δ
packages/apollo-link-state/src/index.ts	93.84% <100%> (+0.19%)	⬆️

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