VR HealthCheck fails when there are no VMs deployed

### problem

A VR HA pair or single VR report the health check as failed when there are no VMs deployed to that VR. This causes alerts in the logs, triggering investigations.

Under the VR health check:
"dhcp_check.py" reports "No VMs running data available, skipping" (which is good)
"webserver.service" reports "warning" "service down at last check xxxxxxx" (which is bad)

This issue goes away if a VM is deployed and returns if it is deleted. If there is an VR HA pair, the failure report bounces back and forth between the 2 VR in the HA pair.

The attached screenshot has r-517/r518 in an HA pair as well as r-519/r-520.

### versions

This is on CloudStack 4.22.

### The steps to reproduce the bug

1. Create a VR Router (HA or not) and an isolated network
2. Deploy a VM to that isolated network
3. Delete the VM
4. The next health check will then report as failed

### What to do about it?

The VR healthcheck should not fail if there are no VMs deployed.

从 dhcp_check.py 和 webserver.service 的 health-check 路径开始，然后在删除其 VM 后，于具有隔离网络的 VR 上复现该问题。完成的标准是：没有已部署 VM 的 VR 仍保持健康，并且不会发出所报告的 failure alert，包括 HA 对的情况。

由索引模型根据 Issue 内容生成。