Allow empty export statements

[alangpierce]

Fixes #335

[codecov-io]

Codecov Report

Merging #338 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #338     +/-   ##
=========================================
+ Coverage   77.05%   77.26%   +0.2%     
=========================================
  Files          41       41             
  Lines        5479     5305    -174     
  Branches     1286     1288      +2     
=========================================
- Hits         4222     4099    -123     
+ Misses        886      836     -50     
+ Partials      371      370      -1

Impacted Files	Coverage Δ
src/CJSImportProcessor.ts	90.43% <100%> (+0.13%)	⬆️
src/transformers/CJSImportTransformer.ts	91.55% <100%> (+0.08%)	⬆️
src/util/getNonTypeIdentifiers.ts	75% <0%> (-25%)	⬇️
src/parser/traverser/util.ts	70.58% <0%> (-13.29%)	⬇️
src/parser/index.ts	75% <0%> (-9.62%)	⬇️
src/parser/util/whitespace.ts	80% <0%> (-7.5%)	⬇️
src/parser/plugins/types.ts	81.25% <0%> (-6.25%)	⬇️
src/computeSourceMap.ts	83.33% <0%> (-5.56%)	⬇️
src/index.ts	84.84% <0%> (-3.79%)	⬇️
src/identifyShadowedGlobals.ts	86.11% <0%> (-1.39%)	⬇️
... and 13 more

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