Handle null value in array of styles.

[tkh44]

const cls1 = css([
  {
    height: 50,
    width: 20
  },
  null
])

[codecov]

Codecov Report

Merging #316 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #316   +/-   ##
=======================================
  Coverage   96.25%   96.25%           
=======================================
  Files          16       16           
  Lines         561      561           
  Branches      137      137           
=======================================
  Hits          540      540           
  Misses         16       16           
  Partials        5        5

Impacted Files	Coverage Δ
packages/emotion/src/index.js	99.02% <100%> (ø)	⬆️

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