fix(filter): string filter should also work when using Contains

[ghiscoding]

• when using OperatorType.empty or OperatorType.contains, they should do a comparison of passing when item contains the substring of search term

[codecov]

Codecov Report

Merging #427 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #427    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          141       141            
  Lines         8855      8856     +1     
  Branches      3059      2932   -127     
==========================================
+ Hits          8855      8856     +1     

Impacted Files	Coverage Δ
...ickgrid/filter-conditions/stringFilterCondition.ts	100.00% <100.00%> (ø)

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