Correctly convert null values to entity properties

Fixes serilog/serilog#620

Author: nblumhardt

src/Serilog.Sinks.AzureTableStorage/Sinks/AzureTableStorageWithProperties/AzurePropertyFormatter.cs

@@ -65,7 +65,7 @@ public static EntityProperty ToEntityProperty(LogEventPropertyValue value, strin
 
 		private static EntityProperty SimplifyScalar(object value)
 		{
-			if (value == null) return null;
+			if (value == null) return new EntityProperty(null);
 
 			var valueType = value.GetType();