BUG/MEDIUM: config: for word expansion, empty or non-existing are the same

BUG/MEDIUM: config: for word expansion, empty or non-existing are the same

Amaury reported a case where "${FOO[*]}" still produces an empty field.
It happens if the variable is defined but does not contain any non-space
characters. The reason is that we special-case word expansion only on
non-existing vars. Let's change the ordering of operations so that word-
expanded vars always pretend the current arg is not an empty quote, so
that we don't make any difference between a non-existing var and an
empty one.

No backport is needed unless commit 1968731765 ("BUG/MEDIUM: config:
solve the empty argument problem again") is.