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

vertiman/node-db-migrate,ViGo5190/node-db-migrate,andela-ooladayo/node-db-migrate,cqricky/node-db-migrate,ramakrishnan/node-db-migrate,wyefei/node-db-migrate,db-migrate/node-db-migrate,MForMarlon/node-db-migrate,apechimp/node-db-migrate,bwiggs/node-db-migrate,snordrapidimages/node-db-migrate,SamuelBolduc/node-db-migrate,marbemac/node-db-migrate,StreamCo/node-db-migrate,db-migrate/node-db-migrate,ror/node-db-migrate,jrdi/node-db-migrate,chatter/node-db-migrate