Fix payload for a single item

Author: francbartoli

pygeoapi/formatter/jsonfg.py

@@ -72,7 +72,10 @@ def write(self, data: dict, options: dict = {}) -> str:
         """
 
         try:
-            fields = list(data["features"][0]["properties"].keys())
+            if data.get("features"):
+                fields = list(data["features"][0]["properties"].keys())
+            else:
+                fields = data["properties"].keys()
         except IndexError:
             LOGGER.error("no features")
             return str()