Fix for rebar3 shell in Erlang 26 when ShellArgs==undefined

[choptastic]

Heh folks!

It seems that #2818 introduced a bug when there are no shell args defined (with Erlang 26), since the default value of ShellArgs in rebar_prv_shell is undefined.

This small fix just handles that situation, since undefined is not a valid value for the 3rd argument of erlang:apply/3.

Thanks!

[ferd]

Hm, I think the better fix would be on line 136 by replacing undefined by []:

rebar3/apps/rebar/src/rebar_prv_shell.erl

Line 136 in 3f82933

ShellArgs = debug_get_value(shell_args, rebar_state:get(State, shell, []), undefined,

I missed it on review because I saw the shell default as [] and forgot there was an extra check in there.

[choptastic]

Thanks, I thought about that as an option as well, but https://github.com/erlang/rebar3/blob/main/apps/rebar/src/rebar_prv_shell.erl#L229 also has as separate check for undefined and I wasn't sure if there would be further cascading regressions for whichever versions of Erlang, so I went with the easy route.

The solution there would be to change that line as well to matching against [] as well, eh?

[ferd]

Yeah I think so, it would make the whole thing more aligned in terms of types.

[choptastic]

Thanks, I've updated the PR. Hopefully it's all good.

[ferd]

Looks good, thanks! will merge as soon as CI passes, which should be unrelated to this since we don't have tests for the shell.

[choptastic]

Thanks @ferd!