Fix test failures in engine_unsafe_empty_string.phpt (#18727)

`/./` matches all characters but newlines, so if `random_bytes`
generates a string with newlines in it, the resulting string is not
empty. Fix this by adding the `s` modifier.

Author: ndossche

ext/random/tests/03_randomizer/engine_unsafe_empty_string.phpt

@@ -11,7 +11,7 @@ final class EmptyStringEngine implements Engine
     public function generate(): string
     {
         // Create a non-interned empty string.
-        return preg_replace('/./', '', random_bytes(4));
+        return preg_replace('/./s', '', random_bytes(4));
     }
 }