[APIM] cannot create an APIM with public network access disabled

Following the documentation, I am trying to issue the following command:

```
az apim create --name "my-private-apim" --publisher-email "" --publisher-name "" --resource-group some-rg-name --location eastus --sku-name Developer --public-network-access false --virtual-network None
```

When I look at the result in the Azure Portal, I can see that the APIM has been provided with a public IP address. When I export the template on azure portal, I can see that the field "publicNetworkAccess" is filled with value "Enabled".

I tried to do the same with option `--virtual-network` set to `Internal` and I also get that the public network access actually is enabled.

Am I doing something wrong? What is happening here?

No source file or test is named. Start by tracing how the APIM create command maps --public-network-access and --virtual-network to the Azure API specification, then reproduce the reported command; done means the resulting setting matches the requested value or the behavior is clearly documented.

Written by the indexing model from the issue text.