c# proto 3 nullable errors with string

[shargon]

Proto3 is nullable by default, but it include null checks in strings and byte[]

protobuf/src/google/protobuf/compiler/csharp/csharp_primitive_field.cc

Lines 269 to 277 in cffa590

	if (is_value_type) {
	printer->Print(
	variables_,
	" $oneof_name$_ = value;\n");
	} else {
	printer->Print(
	variables_,
	" $oneof_name$_ = pb::ProtoPreconditions.CheckNotNull(value, \"value\");\n");
	}