Correct extract_adapters type hint

src/parser.py

@@ -1,5 +1,5 @@
 from glob import glob
-from typing import List
+from typing import List, Union
 
 from pandas import DataFrame, read_html
 
@@ -9,7 +9,7 @@ def find_html_files(path) -> List:
     return file_list
 
 
-def extract_adapters(files) -> DataFrame:
+def extract_adapters(files) -> Union[DataFrame, None]:
     all_adapters = DataFrame()
     for entry in files:
         tables = read_html(entry)