Calico charm generates an invalid CNI configuration file

[Mateo Florido]

The charm generates a `10-calico.conflist` where the `assign_ipv4` and `assign_ipv6` fields contain an invalid value (`null`).

```
{
  "name": "k8s-pod-network",
  "cniVersion": "0.3.1",
  "plugins": [
    {
      "type": "calico",
      "log_level": "info",
      "log_file_path": "/var/log/calico/cni/cni.log",
      "etcd_endpoints": "https://172.30.0.43:2379,https://172.30.0.63:2379,https://172.30.1.153:2379",
      "etcd_key_file": "/etc/cni/net.d/calico-tls/etcd-key",
      "etcd_cert_file": "/etc/cni/net.d/calico-tls/etcd-cert",
      "etcd_ca_cert_file": "/etc/cni/net.d/calico-tls/etcd-ca",
      "mtu": 0,
      "ipam": {
        "type": "calico-ipam",
        "assign_ipv4": null,
        "assign_ipv6": null
      },
      "policy": {
        "type": "k8s"
      },
      "kubernetes": {
        "kubeconfig": "/etc/cni/net.d/calico-kubeconfig"
      }
    },
    {
      "type": "portmap",
      "snat": true,
      "capabilities": {
        "portMappings": true
      }
    },
    {
      "type": "bandwidth",
      "capabilities": {
        "bandwidth": true
      }
    }
  ]
}
```

Also, when attempting to update other configurations in the file (e.g. MTU value), the charm persists in setting the value to 0.

[Mateo Florido (mateoflorido)]

[PR](https://github.com/charmed-kubernetes/charm-calico/pull/103)

[Adam Dyess (addyess)]

Backported to release_1.28 branch