[4.x] Fix compare method callback handling and enforce stricter PHPUnit configuration

- Update `compare` method to properly check for null array keys before determining callback handling logic
- Enable `failOnWarning` and `failOnRisky` in `phpunit.xml` configuration to improve test reliability

Author: andrey-helldar

phpunit.xml

@@ -3,6 +3,8 @@
          xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
          bootstrap="vendor/autoload.php"
          colors="true"
+         failOnWarning="true"
+         failOnRisky="true"
 >
     <testsuites>
         <testsuite name="Test Suite">

src/Benchmark.php

@@ -85,9 +85,9 @@ public function round(?int $precision): self
     public function compare(array|Closure ...$callbacks): static
     {
         $values = match (true) {
-            is_array($callbacks[0]) => $callbacks[0],
-            is_array($callbacks)    => $callbacks,
-            default                 => func_get_args()
+            is_array($callbacks[0] ?? null) => $callbacks[0],
+            is_array($callbacks)            => $callbacks,
+            default                         => func_get_args()
         };
 
         $this->clear();