[csharp] fix "Simplify 'default' expression (IDE0034)" messages #857

Job	Run time
Build .Net projects (samples/client/petstore/csharp/generichost/standard2.0/Petstore/)	2m 51s
Build .Net projects (samples/client/petstore/csharp/httpclient/standard2.0/Petstore/)	1m 50s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/)	2m 21s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/Petstore/)	3m 40s
	10m 42s

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