Fix #82 dry-run-on-first-migration-fails

Just checking that result is defined :
When using `--dry-run`, `PgDriver.runSql` return the callback with no parameters.
As we don't really run the query against the database, we don't have any result.
Merge pull request #132 from jgoyon/fix/dry-run-on-first-migration-fails

Fix #82 dry-run-on-first-migration-fails