fix: guard against preg_replace returning null on malformed UTF-8

preg_replace with /u flag returns null on invalid UTF-8 input. Fall
back to the original text to avoid silently losing input.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: deemonic

src/Core/Analyzer.php

@@ -19,7 +19,7 @@ public function analyze(
 
         // Strip invisible Unicode format characters (zero-width spaces, invisible separators, etc.)
         // before any driver sees the text, ensuring consistent positions across pipeline drivers
-        $text = preg_replace('/\p{Cf}/u', '', $text);
+        $text = preg_replace('/\p{Cf}/u', '', $text) ?? $text;
 
         return $driver->detect($text, $dictionary, $mask, $options);
     }