fix(core): format the pkg.classname as classname in case of empty pkg

[Malinskiy]

Fixes #689

[codecov]

Codecov Report

Merging #692 (a6afe2d) into develop (27eb6dc) will increase coverage by 0.05%.
The diff coverage is 90.00%.

@@              Coverage Diff              @@
##             develop     #692      +/-   ##
=============================================
+ Coverage      62.52%   62.58%   +0.05%     
  Complexity       716      716              
=============================================
  Files            175      175              
  Lines           3560     3565       +5     
  Branches         593      593              
=============================================
+ Hits            2226     2231       +5     
  Misses          1036     1036              
  Partials         298      298              

Impacted Files	Coverage Δ
.../execution/filter/FullyQualifiedClassnameFilter.kt	66.66% <0.00%> (ø)
...rc/main/kotlin/com/malinskiy/marathon/test/Test.kt	92.59% <90.90%> (+1.68%)	⬆️
...skiy/marathon/execution/filter/AllureTestFilter.kt	85.71% <100.00%> (ø)
...n/execution/filter/FullyQualifiedTestnameFilter.kt	100.00% <100.00%> (ø)
...malinskiy/marathon/report/allure/AllureReporter.kt	56.57% <100.00%> (ø)
...inskiy/marathon/report/html/HtmlSummaryReporter.kt	80.70% <100.00%> (ø)
...m/malinskiy/marathon/report/junit/JUnitReporter.kt	87.71% <100.00%> (ø)

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