fix: ak.num should always return a useful (non-unknown length) type
#1655
| Job | Run time |
|---|---|
| 3m 14s | |
| 3m 15s | |
| 3m 4s | |
| 2m 51s | |
| 3m 17s | |
| 2m 22s | |
| 23s | |
| 3m 16s | |
| 2m 28s | |
| 1s | |
| 2m 30s | |
| 3m 4s | |
| 1m 12s | |
| 1s | |
| 3m 10s | |
| 1s | |
| 1s | |
| 1s | |
| 1s | |
| 34m 12s |