empty vs "N/A" vs "DONE (date)" in final status line with mask mode

It may be related to #1372. I am not sure what behaviour is correct, so I'll document my observations.

I tried different masks to tune run time:

• 0 seconds causes final status line without progress indication,
• 0 seconds may give final status line with "N/A",
• 1 -- ~10 seconds give "N/A",
• 10+ seem to give "DONE (date)".

$ cat t4.pw  # crypt.crypt('123456', '$6$asdf')
$6$asdf$PA4y6n7xopdOZJIf7Em3nlPY6ySAAFnKXyHmtvIyajhkoKpzhzDBScEuFqTIsNvR2e4FHA4GTIfNw3FD1CI1o0

$ john --format=sha512crypt --mask='password?d' t4.pw
Using default input encoding: UTF-8
Loaded 1 password hash (sha512crypt, crypt(3) $6$ [SHA512 128/128 SSE4.1 2x])
Cost 1 (iteration count) is 5000 for all loaded hashes
Press 'q' or Ctrl-C to abort, almost any other key for status
0g 0:00:00:00  0g/s 33.33p/s 33.33c/s 33.33C/s password1..password7
Session completed

$ john --format=sha512crypt --mask='password?d?d' t4.pw
[...]
0g 0:00:00:00 N/A 0g/s 204.0p/s 204.0c/s 204.0C/s password95..password77
Session completed

$ john --format=sha512crypt --mask='password?d?d?d?d' t4.pw
[...]
0g 0:00:00:24 DONE (2019-03-30 13:49) 0g/s 409.5p/s 409.5c/s 409.5C/s password9677..password7777
Session completed

$ john --format=sha512crypt --mask='password?d?d?d[0-3]' t4.pw
[...]
0g 0:00:00:10 DONE (2019-03-30 13:50) 0g/s 394.4p/s 394.4c/s 394.4C/s password6573..password7773
Session completed

$ john --format=sha512crypt --mask='password?d?d?d[0-2]' t4.pw
[...]
0g 0:00:00:07 N/A 0g/s 390.1p/s 390.1c/s 390.1C/s password9972..password7772
Session completed

$ john --format=sha512crypt --mask='password?d?d[0-8][0-3]' t4.pw
[...]
0g 0:00:00:08 DONE (2019-03-30 13:51) 0g/s 401.7p/s 401.7c/s 401.7C/s password9683..password7783
Session completed

Reproduce the four john --format=sha512crypt --mask commands from the report and compare their final status lines, especially the differing N/A and DONE output. Determine the expected status behavior for completed mask sessions and verify it consistently across the reported mask lengths and runtimes.