Refactor ConsoleActivityExporter to handle null values in JSON serial…

…ization

src/OpenTelemetry.Exporter.Console/ConsoleActivityExporter.cs

@@ -73,7 +73,11 @@ public override ExportResult Export(in Batch<Activity> batch)
             var options = new JsonSerializerOptions
             {
                 WriteIndented = true,
-                IgnoreNullValues = true,
+                #if NETSTANDARD || NETFRAMEWORK
+                    IgnoreNullValues = true,
+                #else
+                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+                #endif
                 Converters = { new JsonStringEnumConverter(), },
             };