[utils] `sanitize_path`: Fix when path is empty string

yt_dlp/YoutubeDL.py

@@ -2237,7 +2237,7 @@ class YoutubeDL(object):
                         matches = LazyList(_check_formats(matches[::-1 if format_reverse else 1]))
                         try:
                             yield matches[format_idx - 1]
-                        except IndexError:
+                        except LazyList.IndexError:
                             return
 
             filters = [self._build_format_filter(f) for f in selector.filters]

yt_dlp/utils.py

@@ -778,7 +778,7 @@ def sanitize_path(s, force=False):
         for path_part in norm_path]
     if drive_or_unc:
         sanitized_path.insert(0, drive_or_unc + os.path.sep)
-    elif force and s[0] == os.path.sep:
+    elif force and s and s[0] == os.path.sep:
         sanitized_path.insert(0, os.path.sep)
     return os.path.join(*sanitized_path)