Fix emoji: Fix test error by remove "omit empty" on index

[kkdai]

Why need this change / Root cause

Original the index of emoji is omitempty it means if index=0 it will be omitted. But server could not accept any call without index.

Changes made

• Add one more test index=0.
• Remove omit empty on Emoji index.

Verified screenshots (optional)

[codecov-io]

Codecov Report

Merging #209 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #209   +/-   ##
=======================================
  Coverage   76.85%   76.85%           
=======================================
  Files          31       31           
  Lines        2320     2320           
=======================================
  Hits         1783     1783           
  Misses        434      434           
  Partials      103      103           

Impacted Files	Coverage Δ
linebot/emoji.go	100.00% <ø> (ø)

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[nasa9084]

LGTM