[MPG] Generating recoveryservices service results in a build error

[mzhongl524]

Repro:

1. generate code for recoveryservices with new mpg generator

2. the following error occurred while generating the code:

Error : error : Failed to generate the library. Exit code: 1. [D:\Workspace\Repo\azure-sdk-for-net-bak\sdk\recoveryservices\Azure.ResourceManager.RecoveryServices\src\Azure.ResourceManager.Rec
overyServices.csproj]
  StackTrace:
  Object reference not set to an instance of an object.
     at Azure.Generator.Management.Providers.OperationMethodProviders.GetIfExistsOperationMethodProvider.BuildReturnStatements(ScopedApi`1 responseVariable, MethodSignature signature) in /mnt/
  vss/_work/1/s/eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src/Providers/OperationMethodProviders/GetIfExistsOperationMethodProvider.cs:line 41
     at Azure.Generator.Management.Providers.OperationMethodProviders.ResourceOperationMethodProvider.BuildTryExpression() in /mnt/vss/_work/1/s/eng/packages/http-client-csharp-mgmt/generator/
  Azure.Generator.Management/src/Providers/OperationMethodProviders/ResourceOperationMethodProvider.cs:line 215
     at Azure.Generator.Management.Providers.OperationMethodProviders.ResourceOperationMethodProvider.BuildBodyStatements() in /mnt/vss/_work/1/s/eng/packages/http-client-csharp-mgmt/generator
  /Azure.Generator.Management/src/Providers/OperationMethodProviders/ResourceOperationMethodProvider.cs:line 159
     at Azure.Generator.Management.Providers.OperationMethodProviders.ResourceOperationMethodProvider..ctor(TypeProvider enclosingType, RequestPathPattern contextualPath, RestClientInfo restCl
  ientInfo, InputServiceMethod method, Boolean isAsync, String methodName, FormattableString description, Boolean forceLro) in /mnt/vss/_work/1/s/eng/packages/http-client-csharp-mgmt/generator
  /Azure.Generator.Management/src/Providers/OperationMethodProviders/ResourceOperationMethodProvider.cs:line 97
     at Azure.Generator.Management.Providers.OperationMethodProviders.GetIfExistsOperationMethodProvider..ctor(ResourceCollectionClientProvider collection, RequestPathPattern contextualPath, R
  estClientInfo restClientInfo, InputServiceMethod method, Boolean isAsync) in /mnt/vss/_work/1/s/eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src/Providers/Operat
  ionMethodProviders/GetIfExistsOperationMethodProvider.cs:line 24
     at Azure.Generator.Management.Providers.ResourceCollectionClientProvider.BuildGetIfExistsMethods() in /mnt/vss/_work/1/s/eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Man
  agement/src/Providers/ResourceCollectionClientProvider.cs:line 445
     at Azure.Generator.Management.Providers.ResourceCollectionClientProvider.BuildMethods() in /mnt/vss/_work/1/s/eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src
  /Providers/ResourceCollectionClientProvider.cs:line 309
     at Microsoft.TypeSpec.Generator.Providers.TypeProvider.get_Methods()
     at Microsoft.TypeSpec.Generator.Providers.TypeProvider.EnsureBuilt()
     at Microsoft.TypeSpec.Generator.CSharpGen.ExecuteAsync()
     at Microsoft.TypeSpec.Generator.GeneratorRunner.RunAsync(CommandLineOptions options)
     at Microsoft.TypeSpec.Generator.Program.Run(CommandLineOptions options, GeneratorRunner runner)

3. The associated spec file may be： https://github.com/Azure/azure-rest-api-specs/blob/main/specification/recoveryservices/resource-manager/Microsoft.RecoveryServices/RecoveryServices/VaultExtendedInfoResource.tsp