fix error on book without right TOC

src/book.py

@@ -594,7 +594,6 @@ class Book:
 
             footnotes.append(content)
 
-
         self.footnotes = footnotes
 
     def _process_images(self):
@@ -705,6 +704,7 @@ class Book:
         headers_info = []
         header_tags = self.body_tag.find_all(re.compile("^h[1-9]$"))
         headers_outline = [int(re.sub(r"^h", "", tag.name)) for tag in header_tags]
+        if headers_outline:
             top_level_outline = min(headers_outline)
             top_level_headers = [tag for tag in header_tags
                                  if int(re.sub(r"^h", "", tag.name)) == top_level_outline]
@@ -973,6 +973,7 @@ class Book:
 
         # Add is_introduction field to json structure
         # after deleting content before toc, some chapters can be deleted
+        if self.top_level_headers:
             same_first_titles = self.top_level_headers[0]['title'] == json_strc[0]['title']
             is_first_header_introduction = not self.top_level_headers[0]['should_be_numbered']