Prepare for newer PHP versions

[MetalArend]

Bug report

Running PhpMetrics against the newest PHP versions (PHP 8.3) gives a lot of deprecation warnings:

• Creation of dynamic property xxx is deprecated
• Deprecated: abs(): Passing null to parameter #1 ($num) of type int|float is deprecated
• Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated