hush: fix corner cases with exec in empty expansions

hush: fix corner cases with exec in empty expansions

Cases like these:

var=val exec >redir

var=val `` >redir

function                                             old     new   delta
run_pipe                                            1701    1723     +22
redirect_and_varexp_helper                            56      55      -1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 22/-1)              Total: 21 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>