fix documentation generation, avoid error with empty files

--HG-- branch : 15.0

doc/autotodo.py

@@ -101,6 +101,7 @@ def scan_folder(data_tuple, dirname, names):
 
 def scan_file(filename, tags) -> Tuple[Dict[str, Tuple[str, int, str]], int]:
     res = {tag: [] for tag in tags}
+    line_num: int = 0
     with open(filename, "r") as f:
         for line_num, line in enumerate(f):
             for tag in tags: