fix(graphql): pagination offset should never be below zero

[ghiscoding]

No description provided.

[codecov]

Codecov Report

Merging #329 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   97.66%   97.66%   +0.01%     
==========================================
  Files         138      138              
  Lines        7989     7991       +2     
  Branches     2719     2723       +4     
==========================================
+ Hits         7802     7804       +2     
  Misses        186      186              
  Partials        1        1

Impacted Files	Coverage Δ
...s/angular-slickgrid/services/pagination.service.ts	99.35% <100%> (ø)	⬆️
...slickgrid/components/slick-pagination.component.ts	100% <100%> (ø)	⬆️
...ules/angular-slickgrid/services/graphql.service.ts	100% <100%> (ø)	⬆️

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