Fix NewRowSynthesis on datetime columns without formats
#1407
| Job | Run time |
|---|---|
| 5m 20s | |
| 5m 53s | |
| 14m 52s | |
| 6m 19s | |
| 6m 17s | |
| 11m 4s | |
| 4m 59s | |
| 7m 38s | |
| 12m 23s | |
| 6m 12s | |
| 5m 17s | |
| 12m 52s | |
| 1h 39m 6s |