db_postgres.getValue issues warning when no rows found

When calling `getValue` in db_postgres,  the underlying Postgres library issues a warning when there are no rows that were found by the SELECT statement. 

The fix is to check pqntuples() is greater than 0 before calling pqgetvalue.  This is similar to #11821 

```
$ nim -v
Nim Compiler Version 1.0.4
```
