test: Add (failing) test for nulls in frequencies_as_map

Author: colinbarry

e2e/collections_test/test_frequencies_as_map4/input.cyp

@@ -0,0 +1,5 @@
+query: >
+  RETURN collections.frequencies_as_map([1, 1, null, null, 1]) AS result;
+
+output:
+  - result: {"1": 3, "null": 2}