Fix undefined config error with responsive-loader set as default

[christinecha]

Currently, if you use defaultImageLoader: 'responsive-loader' in your config, your next build will error out because there is no return value at the end of applyResponsiveLoader, so your whole webpack/next config becomes undefined.

This should fix that!

[codecov]

Codecov Report

Merging #73 into master will decrease coverage by 0.54%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
- Coverage   92.89%   92.35%   -0.55%     
==========================================
  Files          13       13              
  Lines         169      170       +1     
  Branches       40       40              
==========================================
  Hits          157      157              
- Misses          9       10       +1     
  Partials        3        3

Impacted Files	Coverage Δ
lib/loaders/responsive-loader.js	72.72% <0%> (-7.28%)	⬇️

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[cyrilwanner]

Oops, I totally missed this. Thank you @christinecha very much for this bug fix! I'll publish it to NPM in a few minutes.