Fix test code for PHPStan level 9

[Art4]

Fix #74.

[codecov-commenter]

Codecov Report

Merging #85 (901f121) into v1.x (936faba) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##                v1.x       #85   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       288       295    +7     
===========================================
  Files             29        29           
  Lines            615       625   +10     
===========================================
+ Hits             615       625   +10     

Files	Coverage Δ
src/Helper/AccessKey.php	100.00% <ø> (ø)
src/Helper/AccessableTrait.php	100.00% <100.00%> (ø)
src/Serializer/ArraySerializer.php	100.00% <ø> (ø)
src/V1/Attributes.php	100.00% <ø> (ø)
src/V1/Document.php	100.00% <ø> (ø)
src/V1/DocumentLink.php	100.00% <ø> (ø)
src/V1/Error.php	100.00% <ø> (ø)
src/V1/ErrorCollection.php	100.00% <ø> (ø)
src/V1/ErrorLink.php	100.00% <ø> (ø)
src/V1/ErrorSource.php	100.00% <ø> (ø)
... and 12 more

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