Portal does not accept an empty value for string parameters

When creating an environment from an ARM template using the DevTest Labs portal, I get error `The value must not be empty` if I pass an empty value for a string parameter.

This example allows to reproduce the issue:

```json
{
"$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {
"incomingIPAllowedForRDP": {
"type": "string",
"defaultValue": "*",
"minLength": 0,
"metadata": {
"description": "Specify which IP addresses are allowed to use RDP to connect to the VMs:
- If empty: Firewall denies all incoming RDP traffic from Internet.
- If '*' (default): Firewall accepts all incoming RDP traffic from Internet.
- If 'IPAddress': Firewall accepts incoming RDP traffic only from the specified 'IPAddress'."
}
}
},
"variables": {
},
"resources": [
],
"outputs": {
}
}
```

More info:
- Passing an empty string using works fine
- Passing an empty string using `New-AzResourceGroupDeployment` works fine