BUG: Check for AcroForm and ensure it is not None (#1965)

Author: nickryand

pypdf/_writer.py

@@ -2921,7 +2921,7 @@ def merge(
                     pag[NameObject("/Annots")] = lst
                 self.clean_page(pag)
 
-        if "/AcroForm" in cast(DictionaryObject, reader.trailer["/Root"]):
+        if "/AcroForm" in _ro and _ro['/AcroForm'] is not None:
             if "/AcroForm" not in self._root_object:
                 self._root_object[NameObject("/AcroForm")] = self._add_object(
                     cast(