Remove nullable type from exceptionIsFlaky method

The method is only called after null is already checked by the caller,
so the nullable type hint was misleading and could cause null dereference
if called directly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: aarondfrancis

src/Flaky.php

@@ -235,7 +235,7 @@ protected function shouldThrowImmediately(?Throwable $exception = null): bool
         return $this->protectionsBypassed() || !$this->exceptionIsFlaky($exception);
     }
 
-    protected function exceptionIsFlaky(?Throwable $exception = null): bool
+    protected function exceptionIsFlaky(Throwable $exception): bool
     {
         return $this->flakyExceptions === null || in_array($exception::class, $this->flakyExceptions, true);
     }