feat: use sqlx instead of sql (#79)

This PR updates the `PostgreSQL` database to use the sqlx driver instead of `sql` to provide more functionalities while maintaining backward compatibility

- [x] Targeted PR against correct branch.
- [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [ ] Wrote unit tests.
- [x] Re-reviewed `Files changed` in the Github PR explorer.

database/postgresql/postgresql.go

@@ -105,6 +105,10 @@ func (db *Database) GetLastBlockHeight() (int64, error) {
 		return 0, fmt.Errorf("error while getting last block height, error: %s", err)
 	}
 
+	if height == 0 {
+		return 0, fmt.Errorf("cannot get block height, no blocks saved")
+	}
+
 	return height, nil
 }