certctl(8): Pacify formal POSIX sh syntax.

According to POSIX 2018, the syntax between `then' and `elif' and
`fi' must be a _non-empty_ list of commands:

compound_list    : linebreak term
                | linebreak term separator
                ;
...
if_clause        : If compound_list Then compound_list else_part Fi
                | If compound_list Then compound_list          Fi
                ;
else_part        : Elif compound_list Then compound_list
                | Elif compound_list Then compound_list else_part
                | Else compound_list
                ;

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_10_02

NetBSD's sh(1) currently doesn't enforce this and allows an empty
sequence of commands, but let's not rely on that nonstandard quirk.

Noted in PR 57997.

(riastradh)