fix: ak.num should always return a useful (non-unknown length) type
#1671
| Job | Run time |
|---|---|
| 5m 20s | |
| 3m 17s | |
| 5m 20s | |
| 5m 48s | |
| 2m 36s | |
| 4m 39s | |
| 4m 31s | |
| 5m 29s | |
| 5m 33s | |
| 2m 26s | |
| 2m 29s | |
| 4m 48s | |
| 5m 17s | |
| 5m 34s | |
| 4m 22s | |
| 4m 23s | |
| 4m 16s | |
| 7m 12s | |
| 1h 23m 20s |