bin/57997: sh(1) allows an empty list in if statements

[rvp%SDF.ORG@localhost]

>Number:         57997
>Category:       bin
>Synopsis:       sh(1) allows an empty list in if statements
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Mar 04 18:30:01 +0000 2024
>Originator:     RVP
>Release:        NetBSD-10.99.10
>Organization:
>Environment:
NetBSD CoreBook.local 10.99.10 NetBSD 10.99.10 (MYKERNEL) #0: Sun Mar  3 03:37:44 UTC 2024  bld@CoreBook.local:/tmp/obj/usr/src/sys/arch/amd64/compile/MYKERNEL amd64
>Description:
Apparently, NetBSD's sh(1) doesn't need a `list' after `then' in `if'
statements:

```
#!/bin/sh

if      false
then
elif    false
then
else
fi
echo "Should not reach this!"
```

NetBSD's /bin/sh first here, then some other shells:

```
$ ./then.sh
Should not reach this!

$ bash ./then.sh
./then.sh: line 5: syntax error near unexpected token `elif'
./then.sh: line 5: `elif        false'

$ ksh ./then.sh
./then.sh[5]: syntax error: `elif' unexpected
./then.sh[5]: 

$ ksh93 ./then.sh
./then.sh: syntax error at line 5: `elif' unexpected

$ dash ./then.sh
./then.sh: 5: Syntax error: "elif" unexpected

```

This is the case even in NetBSD 9.3. Haven't checked 8.x. First noticed
in the rehash() function of /usr/sbin/certctl.
>How-To-Repeat:
As above.
>Fix:
No clue.