Do not pass null to ReflectionClass::getMethods()

[erdemkose]

Fixes #5016

PHP <7.2.18 and PHP <7.3.5 throw error when null is passed to ReflectionClass::getMethods() when strict_types is enabled.

[codecov]

Codecov Report

Merging #5026 (b276edb) into 8.5 (9edbd2e) will increase coverage by 81.94%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             8.5    #5026       +/-   ##
==========================================
+ Coverage       0   81.94%   +81.94%     
- Complexity     0     5701     +5701     
==========================================
  Files          0      588      +588     
  Lines          0    14945    +14945     
==========================================
+ Hits           0    12246    +12246     
- Misses         0     2699     +2699     

Impacted Files	Coverage Δ
src/Util/Reflection.php	100.00% <100.00%> (ø)
src/TextUI/Configuration/TestSuiteBuilder.php	100.00% <0.00%> (ø)
...ct/Exception/MethodCannotBeConfiguredException.php	0.00% <0.00%> (ø)
src/Event/TypeMap.php	54.54% <0.00%> (ø)
src/Metadata/PreserveGlobalState.php	100.00% <0.00%> (ø)
...rc/TextUI/Command/Commands/VersionCheckCommand.php	0.00% <0.00%> (ø)
src/Metadata/TestDox.php	100.00% <0.00%> (ø)
...ictAboutResourceUsageDuringSmallTestsAttribute.php	75.00% <0.00%> (ø)
src/Event/Value/TestData/TestDataCollection.php	65.21% <0.00%> (ø)
...ner/TestResult/Subscriber/TestFailedSubscriber.php	100.00% <0.00%> (ø)
... and 579 more

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