Check for null when evaluating input strings in createStyles

[tkh44]

closes #351

[codecov]

Codecov Report

Merging #352 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #352   +/-   ##
=======================================
  Coverage   97.36%   97.36%           
=======================================
  Files          17       17           
  Lines         569      569           
  Branches      134      134           
=======================================
  Hits          554      554           
  Misses         11       11           
  Partials        4        4

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

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