Allow newer phpunit versions and update docblocks

[mmachatschek]

Purpose

• Allow newer PHPUnit versions.
• Use <Object>[] syntax in docblock.

Open Questions and Pre-Merge TODOs

• composer lint and composer fix was executed.
• Tests were written and pass with 100% coverage.

[codecov-io]

Codecov Report

Merging #118 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             develop   #118   +/-   ##
========================================
  Coverage        100%   100%           
  Complexity       269    269           
========================================
  Files             49     49           
  Lines            701    700    -1     
========================================
- Hits             701    700    -1

Impacted Files	Coverage Δ	Complexity Δ
src/FINDOLOGIC/Export/Exporter.php	100% <ø> (ø)	5 <0> (ø)	⬇️
src/FINDOLOGIC/Export/XML/Page.php	100% <ø> (ø)	9 <0> (ø)	⬇️
src/FINDOLOGIC/Export/XML/XMLItem.php	100% <ø> (ø)	26 <0> (ø)	⬇️
...rt/Exceptions/EmptyElementsNotAllowedException.php	100% <100%> (ø)	1 <0> (ø)	⬇️

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