Instance mix create returns NoneType exception

### **This is autogenerated. Please review and update as needed.**

## Describe the bug
Trying to create vmss with instance mix

**Command Name**
`az vmss create`

**Errors:**
```
'NoneType' object has no attribute 'get'
```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az vmss create --resource-group {} --name {} --image {} --instance-count {} --vm-sku {} --skuprofile-vmsizes {} {} {} --skuprofile-allocation-strategy {} --zones {} {} {} --orchestration-mode {} --platform-fault-domain-count {} --admin-username {} --generate-ssh-keys --output {}`

## Expected Behavior
successful create of vmss

## Environment Summary
```
Windows-11-10.0.26100-SP0
Python 3.12.10
Installer: MSI

azure-cli 2.74.0

Extensions:
kusto 0.5.0

Dependencies:
msal 1.32.3
azure-mgmt-resource 23.3.0
```
## Additional Context