diff: fix empty string handling

[matej21]

Currently comparing with an empty strings throws following error:

TypeError: Cannot read property 'count' of undefined
    at chars (/..../node_modules/uvu/diff/index.js:95:36)

[codecov-io]

Codecov Report

Merging #59 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #59   +/-   ##
=======================================
  Coverage   90.16%   90.16%           
=======================================
  Files           3        3           
  Lines         305      305           
=======================================
  Hits          275      275           
  Misses         30       30           

Impacted Files	Coverage Δ
src/diff.js	100.00% <100.00%> (ø)

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[lukeed]

Ah, thank you!

[lukeed]

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