Fix some errors found in calc examples

desktop_env/evaluators/metrics/pdf.py

@@ -6,6 +6,8 @@ from pypdf import PdfReader
 
 
 def check_pdf_pages(pdf_file: str, rules: Dict[str, Any]) -> float:
+    if pdf_file is None:
+        return 0.0
     reader = PdfReader(pdf_file)
     nb_pages: int = len(reader.pages)
     return float(getattr(operator, rules["relation"])(nb_pages, rules["ref_value"]))