"null" value for description

Getting "null" value for the description when trying to add rule using "az network firewall policy rule-collection-group collection rule" command.

Reproduced and tried multiple times with the same result.

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#### Document Details

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* ID: 51bc8847-2744-bb66-7b83-0518aa47536c
* Version Independent ID: c1c31748-e899-dd98-e418-e01ed781a14a
* Content: [az network firewall policy rule-collection-group collection rule](https://docs.microsoft.com/en-us/cli/azure/network/firewall/policy/rule-collection-group/collection/rule?view=azure-cli-latest#az_network_firewall_policy_rule_collection_group_collection_rule_add)
* Content Source: [latest/docs-ref-autogen/network/firewall/policy/rule-collection-group/collection/rule.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/network/firewall/policy/rule-collection-group/collection/rule.yml)
* Service: **virtual-network**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**