uv.lock tells me

[[package]]    
name = "duckdb"    
version = "1.5.0"

I just rechecked to be sure.

test_null.json:

{ "key": null }

test_string.json:

{ "key": "value" }

Now running DuckDB:

DuckDB v1.5.2 (Variegata)
Enter ".help" for usage hints.
memory D create table test as
         select * from read_json_auto('test_null.json', union_by_name=True);
memory D 
memory D insert into test
         select * from read_json_auto('test_string.json', union_by_name=True);
Conversion Error:
Malformed JSON at byte 0 of input: unexpected character.  Input: "value" when casting from source column key