[services-geojson] NullPointerException in Feature.addXxxProperty() or getXxxProperty() methods

All methods like addStringProperty(), addNumberProperty(), getStringProperty()… don’t guard against a null properties field.

The “guard” is at a higher level, as using Feature static factory methods ensure that the properties field is never null.
However Feature.GsonTypeAdapter.read() directly calls the Feature constructor which doesn’t prevent a null properties field. So for example, if someone calls FeatureCollection.fromJson() which contains a feature that has a null properties field, it will be decoded by the GsonAdapter and then calling addStringProperty() on this feature will crash.

Small code sample to demonstrate that

public void addPropertyCrash() {
    final String json = "{" +
            "\"type\": \"FeatureCollection\"," +
            "\"features\": [" +
                "{" +
                    "\"type\": \"Feature\"," +
                    "\"properties\": null," +
                    "\"geometry\": null" +
                "}" +
            "]" +
            "}";

    FeatureCollection actual = FeatureCollection.fromJson(json);
    actual.features().get(0).addStringProperty("key", "value");
}

從 FeatureCollection.fromJson() 呼叫到的 Feature.GsonTypeAdapter.read() 開始，檢查 null 的 properties 欄位如何到達 Feature.addStringProperty() 及相關的 add/get 方法。驗證提供的 FeatureCollection JSON 範例在新增屬性時不再導致 NullPointerException。