revert: revert "return 0 height inside GetLastBlockHeight if there ar…

…e no blocks" (#78)

## Description

fix #76 
I forgot to remove this section of codes lol

## Checklist
- [ ] 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.  
- [ ] Re-reviewed `Files changed` in the Github PR explorer.

database/postgresql/postgresql.go

@@ -105,10 +105,6 @@ 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
 }