fix(fingerprint): return unique fallback hash instead of null when element is missing

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

Author: Abhishek Bindra

lib/core/utils/dq-element.js

@@ -346,9 +346,9 @@ const DqElement = memoize(function DqElement(elm, options, spec) {
   try {
     this._htmlHash = this._element
       ? computeFingerprintHash(this._element.outerHTML)
-      : null;
+      : computeFingerprintHash(null);
   } catch {
-    this._htmlHash = null;
+    this._htmlHash = computeFingerprintHash(null);
   }
 
   /**