Fix field not found when alias field resolve falsy

[trungtin]

When aliasedNode resolve falsy value (0, false, null) and preAliasingNode resolve undefined then the error field not found appears, this should fixes it.

[apollo-cla]

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[trungtin]

close as duplicate of #303

[codecov-io]

Codecov Report

Merging #337 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #337      +/-   ##
==========================================
+ Coverage      95%   95.12%   +0.12%     
==========================================
  Files           2        2              
  Lines          80       82       +2     
  Branches       22       22              
==========================================
+ 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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