[csharp] fix "Simplify 'default' expression (IDE0034)" messages (#21147) #858

Job	Run time
Build .Net projects (samples/client/petstore/csharp/httpclient/standard2.0/Petstore/)	3m 33s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/Petstore/)	3m 35s
Build .Net projects (samples/client/petstore/csharp/generichost/standard2.0/Petstore/)	3m 35s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/)	3m 35s
	14m 18s

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