[I] `MERGE` creates incomplete vertices if the `RETURN` clause does not include the created vertices [age]

[via GitHub]

mingfang opened a new issue, #1709:
URL: https://github.com/apache/age/issues/1709

   when creating multiple vertices like this
   ```sql
   SELECT * FROM cypher('playground', $$
     UNWIND [{first: 'jon', last: 'snow'}, {first: 'ned', last: 'stark'}] AS map
     MERGE (v:PERSON {first: map.first})
     SET v=map
     RETURN count(v)
   $$) AS (v agtype)
   ```
   returns count of 2, but all the vertices after the first one will be 
incomplete. e.g.
   ```sql
   SELECT * FROM cypher('playground', $$
     MATCH (v:PERSON)                                                           
     RETURN v
   $$) AS (v agtype)
   ```
   returns
   ```shell
   {"id": 3940649673949198, "label": "PERSON", "properties": {"last": "snow", 
"first": "jon"}}::vertex
   {"id": 3940649673949199, "label": "PERSON", "properties": {"first": 
"ned"}}::vertex
   ```
   Note the first vertex is correct but the second vertex is missing the "last" 
field.  Very strange.
   
   But it works if I change the `RETURN` clause to include the vertex variable 
like this
   
   ```sql
   SELECT * FROM cypher('playground', $$
     UNWIND [{first: 'jon', last: 'snow'}, {first: 'ned', last: 'stark'}] AS map
     MERGE (v:PERSON {first: map.first})
     SET v=map
     RETURN v
   $$) AS (v agtype)
   ```
   works correctly and returns
   ```shell
   {"id": 3940649673949196, "label": "PERSON", "properties": {"last": "snow", 
"first": "jon"}}::vertex
   {"id": 3940649673949197, "label": "PERSON", "properties": {"last": "stark", 
"first": "ned"}}::vertex
   ```


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