Fix: Test data providers directly

[localheinz]

This PR

• tests data providers directly

Follows #326.

[codecov]

Codecov Report

Merging #332 into main will increase coverage by 31.02%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##               main      #332       +/-   ##
==============================================
+ Coverage     68.97%   100.00%   +31.02%     
  Complexity       65        65               
==============================================
  Files             8         8               
  Lines           245       227       -18     
==============================================
+ Hits            169       227       +58     
+ Misses           76         0       -76     

Impacted Files	Coverage Δ	Complexity Δ
src/DataProvider/NullProvider.php	100.00% <0.00%> (+100.00%)	1.00% <0.00%> (ø%)
src/DataProvider/StringProvider.php	100.00% <0.00%> (+100.00%)	8.00% <0.00%> (ø%)
src/DataProvider/BooleanProvider.php	100.00% <0.00%> (+100.00%)	4.00% <0.00%> (ø%)

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