Route.Upstream.Retries field needs to support zero value

[Chaunceyan]

Issue description

As shown in the doc, the retries field expected zero and null value to behave differently.
But current entity definition omit zero value to empty, so this configuration does not function properly.

I am raising PR for this.

Expected behavior

Zero and Null value should be well distinguished.

How to Reproduce

1. Create Upstream with retries being zero.
2. View Upstream to see the retries being empty.

Screenshots

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

[zaunist]

@juzhiyuan . We can close this issue after the #2298 merged.