fix: check undefined string and show how to correct

[seongwon-kang]

Fixes #210

[codecov]

Codecov Report

Merging #211 (60de0ac) into master (d3f7042) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #211   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        30           
  Lines          121       123    +2     
  Branches        31        32    +1     
=========================================
+ Hits           121       123    +2     

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

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