switch to empty string

app.py

@@ -25,7 +25,7 @@ def convert_file_to_pdf(filename:str) -> str:
     converter.convert(filename.name, output_path)
     return output_path
 
-def convert_file_to_img(image_file:str=None,txt:str=None) -> List[Image.Image] :
+def convert_file_to_img(image_file:str="",txt:str="") -> List[Image.Image] :
     """
     Convert an image file to PDF format.
 
@@ -39,9 +39,9 @@ def convert_file_to_img(image_file:str=None,txt:str=None) -> List[Image.Image] :
              the same as the input filename but with a .pdf extension.
     """
     img_list = []
-    if txt is not None: 
+    if txt != "": 
         image_file.append(load_img(txt))
-    if image_file is not None : 
+    if image_file != "" : 
         output_path = image_file.name.rsplit('.', 1)[0] + '.pdf'
         converter.convert(image_file.name, output_path)
         doc = fitz.open(output_path)