Fix error when no "targets" options in .babelrc

[starkwang]

When .babelrc has no "targets" option in env, like:

{
  "presets": ["env"]
}

Then it will cause an error:

Cannot read property 'targets' of undefined
    at loadBabelrc (/node_modules/parcel-bundler/src/utils/getTargetEngines.js:113:23)
    at <anonymous>

This PR is to fix it.

[codecov-io]

Codecov Report

Merging #886 into master will decrease coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #886      +/-   ##
==========================================
- Coverage   88.92%   88.77%   -0.15%     
==========================================
  Files          68       68              
  Lines        3440     3440              
==========================================
- Hits         3059     3054       -5     
- Misses        381      386       +5

Impacted Files	Coverage Δ
src/utils/getTargetEngines.js	96.62% <100%> (ø)	⬆️
src/assets/CSSAsset.js	81.73% <0%> (-4.35%)	⬇️
src/assets/StylusAsset.js	82.27% <0%> (-1.27%)	⬇️
src/visitors/dependencies.js	83.11% <0%> (+1.29%)	⬆️

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