Plugin with empty json config is dropped when add or edit Route

[wudesheng]

Issue description

if you add a plugin with empty config when add or edit route, the plugin is not saved.

Expected behavior

key-auth plugin saved

How to Reproduce

1. add/edit route
2. add a key-auth plugin with empty config
3. save
4. view route
5. the key-auth plugin is not saved.

Screenshots

No response

Environment

• apisix version (cmd: apisix version):
• OS (cmd: uname -a):
• OpenResty / Nginx version (cmd: nginx -V or openresty -V):
• etcd version, if have (cmd: run etcd --version):
• apisix-dashboard version, if have:
• Browser version, if have:

Additional context

No response

[DSingh0304]

Found the issue the fast-clean library removes empty objects by default, so plugins with empty config like key-auth get dropped before saving. Working on a fix.