az spring-cloud app deployment create always fails with "validation error: Parameter 'deployment_name' can not be None." on new infra

- 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)

### Extension name (the extension in question)
spring-cloud

### Description of issue (in as much detail as possible)

When running the command az spring-cloud app deployment create, I always get "validation error: Parameter 'deployment_name' can not be None." on new infra.

Workaround: use the flag --skip-clone-settings

It looks like it's not handling the case where there are no active deployments on L628 here:
https://github.com/Azure/azure-cli-extensions/blob/master/src/spring-cloud/azext_spring_cloud/custom.py