fix: default avatar missing after signup in non-kiosk mode

matemat/webserver/pagelets/signup.py

@@ -35,7 +35,9 @@ def signup_user(args: FormsDict, files: FormsDict, db: MatematDatabase) -> User:
     # Finally, set the avatar, if provided
     if 'avatar' in files:
         avatar = files.avatar.file.read()
-        if len(avatar) > 0:
+    else:
+        avatar = None
+    if avatar:
         filemagic: magic.FileMagic = magic.detect_from_content(avatar)
         if filemagic.mime_type.startswith('image/'):
             abspath: str = os.path.join(os.path.abspath(config['UploadDir']), 'thumbnails/users/')