change div operastor to return null on division by zero

this changes 

```SQL
SELECT 'a=>3, b=>2, c=>5'::inthstore / 'a=>2, b=>2, c=>0'::floathstore;
                               ?column?                               
 ---------------------------------------------------------------------
  "a"=>"1.5000000000000000", "b"=>"1.00000000000000000000", "c"=>NULL
 (1 row)
```

to return a NULL value in the result instead raising `division by zero`