make Feature.properties optional

* up to now it was required, but could be null

src/GeoJSON.Net/Feature/Feature.cs

@@ -39,7 +39,7 @@ public Feature(TGeometry geometry, TProps properties, string id = null)
         [JsonConverter(typeof(GeometryConverter))]
         public TGeometry Geometry { get; }
 
-        [JsonProperty(PropertyName = "properties", Required = Required.AllowNull)]
+        [JsonProperty(PropertyName = "properties", Required = Required.Default)]
         public TProps Properties { get; }
 
         /// <summary>