Handle null elements in getData

[methyl]

Null is valid element in React, so let's handle it and return no data in this case.

Issue: When rendering null inside story with addWithInfo, exception is thrown.

How to test

storiesOf('Test').addWithInfo('test', '', () => <div>{null}</div>)

[ndelangen]

Thanks @methyl, I will merge this !

[codecov]

Codecov Report

Merging #926 into master will decrease coverage by <.01%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #926      +/-   ##
==========================================
- Coverage   26.88%   26.87%   -0.01%     
==========================================
  Files         192      192              
  Lines        4375     4380       +5     
  Branches      700      701       +1     
==========================================
+ Hits         1176     1177       +1     
- Misses       3199     3203       +4

Impacted Files	Coverage Δ
packages/addon-info/src/components/Node.js	21.25% <25%> (-0.09%)	⬇️

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