fix(Pagination): set a default activePage in Pagination's state

[gcalica]

Fixes #4035

On the first render of a Pagination, if there was no activePage or defaultActivePage value specified, activePage is undefined for "Previous Item" and "Next Item" PaginationItems. This PR sets an initial state for activePage.

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[codecov-commenter]

Codecov Report

Merging #4037 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4037   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files         183      183           
  Lines        3275     3276    +1     
=======================================
+ Hits         3270     3271    +1     
  Misses          5        5           

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

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[layershifter]

Thanks for fixing this ❤️

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉