fix: Update the Description of map_top_n (facebookincubator#12216)

Summary:
Pull Request resolved: facebookincubator#12216

Update the description of map_top_n

map function **map_top_n** returns a new map containing only the top N elements, sorted in descending order by value. In cases where values are tied, the function sorts the tied elements in lexicographical descending order by key.

Since both 'a' and 'c' have null values, 'c' is included in the top 2 results along with 'b',

Reviewed By: zacw7

Differential Revision: D68913230

fbshipit-source-id: 7d0483d28e2f7eada5e1dcb7e0bd000857f831ac

velox/docs/functions/presto/map.rst

@@ -112,7 +112,7 @@ Map Functions
     ``n`` must be a non-negative BIGINT value.::
 
         SELECT map_top_n(map(ARRAY['a', 'b', 'c'], ARRAY[2, 3, 1]), 2) --- {'b' -> 3, 'a' -> 2}
-        SELECT map_top_n(map(ARRAY['a', 'b', 'c'], ARRAY[NULL, 3, NULL]), 2) --- {'b' -> 3, 'a' -> NULL}
+        SELECT map_top_n(map(ARRAY['a', 'b', 'c'], ARRAY[NULL, 3, NULL]), 2) --- {'b' -> 3, 'c' -> NULL}
 
 .. function:: map_top_n_keys(map(K,V), n) -> array(K)
 

velox/functions/prestosql/tests/MapTopNTest.cpp

@@ -129,6 +129,16 @@ TEST_F(MapTopNTest, equalValues) {
   auto expectedResults = makeMapVector({0, 3}, expectedKeys, expectedValues);
 
   assertEqualVectors(expectedResults, result);
+
+  auto dataWithStrKey =
+      makeRowVector({makeMapVectorFromJson<std::string, int64_t>(
+          {R"({"a":2, "b":3, "c":1})", R"({"a":null, "b":3, "c":null})"})});
+
+  auto resultWithStrKey = evaluate("map_top_n(c0, 2)", dataWithStrKey);
+
+  auto expectedWithStrKey = makeMapVectorFromJson<std::string, int64_t>(
+      {R"({"b":3, "a":2})", R"({"b":3, "c":null})"});
+  assertEqualVectors(expectedWithStrKey, resultWithStrKey);
 }
 
 TEST_F(MapTopNTest, timestampWithTimeZone) {