[TCGC] Add decorator to represent models-to-treat-empty-string-as-null configuration for csharp emitter

[live1206]

Clear and concise description of the problem

https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicefabricmanagedclusters/Azure.ResourceManager.ServiceFabricManagedClusters/src/autorest.md

models-to-treat-empty-string-as-null:
  - ManagedClusterSubnet
  - NodeTypeFrontendConfiguration
  - ServiceFabricManagedClusterData
  - ServiceFabricManagedClusterVersion
  - ServiceFabricManagedNodeTypeData
  - VmManagedIdentity

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the TypeSpec repo
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• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[tadelesh]

if it is a language specific behavior, i doubt if it should be a tcgc decorator.

[live1206]

if it is a language specific behavior, i doubt if it should be a tcgc decorator.

Think we had a discussion about this in #1229 already, we put csharp specific decorator in TCGC.

[iscai-msft]

can we talk about this in our dpg sync on wednesday before you guys merge this? Thanks!