`az containerapp update` is not setting minreplicas to 0

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Related command
`containerapp update`

### Extension name (the extension in question)
containerapp

### Description of issue (in as much detail as possible)
I originally created a containerapp with a yaml file that specified `minReplicas: 0`

Now, I want to update minReplicas to 0 so I updated the yaml file like this:
```
scale:
minReplicas: 0
maxReplicas: 1
```

and ran

`az container app update ... --yaml updated.yaml`

However, in the output from the command and in the portal, the minReplicas is still set to 0.

Then, I tried the following:

`az containerapp update ... --min-replicas 0`

The command completed successfully, but minReplicas did not get updated.

Final try - I updated minReplicas via a Revision in the Portal and that finally worked.

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