#37 Add isEmpty into mappers

[Gontrum]

Hi everyone.
This is my first first post and I hope everything is ok so far.
If something does not fit, please give me feedback.

Closes #37

• created isEmpty with logic provided in tests
• exported isEmpty in index.ts
• added tests for isEmpty
• added Example for isEmpty into README
• listed isEmpty in mappers section of readme

[codecov]

Codecov Report

Merging #41 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #41   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        18    +1     
  Lines          279       286    +7     
  Branches         5         5           
=========================================
+ Hits           279       286    +7     

Impacted Files	Coverage Δ
src/mappers/index.ts	100.00% <100.00%> (ø)
src/mappers/isEmpty.ts	100.00% <100.00%> (ø)
tests/unit/mappers/stringMapper.test.ts	100.00% <100.00%> (ø)

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[Siemienik]

Everything looks good for me :) Thank you for your first contribution here 👍