Fixed error can be present when description is None

Author: eunwoo1104

.gitignore

@@ -2,4 +2,5 @@
 __pycache__
 test.py
 test2.py
-docs/_build
+docs/_build
+slash.log

discord_slash/client.py

@@ -103,7 +103,7 @@ def add_slash_command(self,
         name = name.lower()
         _cmd = {
             "func": cmd,
-            "description": description,
+            "description": description if description else "No description.",
             "auto_convert": auto_convert,
             "guild_ids": guild_ids,
             "api_options": options if options else [],