Empty list memory optimization

[stefanhaustein]

Don't allocate a list object for empty arrays; use Collections.emptyList() instead.

[codecov]

Codecov Report

Merging #201 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #201   +/-   ##
=======================================
  Coverage   27.93%   27.93%           
=======================================
  Files          22       22           
  Lines         809      809           
  Branches       88       89    +1     
=======================================
  Hits          226      226           
  Misses        565      565           
  Partials       18       18

Impacted Files	Coverage Δ
...rc/main/java/com/vanniktech/emoji/emoji/Emoji.java	88.57% <100%> (ø)	⬆️

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