add empty lists support for built-in function avg()

Author: sanyer

jmespath/functions.py

@@ -166,7 +166,10 @@ def _func_abs(self, arg):
 
     @signature({'types': ['array-number']})
     def _func_avg(self, arg):
-        return sum(arg) / float(len(arg))
+        if arg:
+            return sum(arg) / float(len(arg))
+        else:
+            return None
 
     @signature({'types': [], 'variadic': True})
     def _func_not_null(self, *arguments):

tests/compliance/functions.json

@@ -83,6 +83,10 @@
       "expression": "avg(strings)",
       "error": "invalid-type"
     },
+    {
+      "expression": "avg(empty_list)",
+      "result": null
+    },
     {
       "expression": "ceil(`1.2`)",
       "result": 2