nl: allow zero for --join-blank-lines

[cakebaker]

This PR allows 0 as value for --join-blank-lines to match the behavior of GNU nl:

$ printf "\n\n\n\n\n\n" | nl --join-blank-lines=0 --body-numbering=a
     1
     2
     3
     4
     5
     6

So far we failed with an "invalid number" error.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)