Parse slices even for empty variables

When a variable is parsed as being empty, parse out the slice and
validate the indexes anyway, behaving for slicing purposes as if the
variable had a single empty value.

Besides providing errors when expected, this also fixes the following:

    set -l foo
    echo "$foo[1]"

This used to print "[1]", now it properly prints nothing.

1 files changed, 12 insertions, 0 deletions

diff --git a/tests/expansion.in b/tests/expansion.in
index d362e789..8b48772e 100644
--- a/tests/expansion.in
+++ b/tests/expansion.in
@@ -62,3 +62,15 @@ show "$$foo"
 show $$foo
 show "prefix$$foo"
 show prefix$$foo
+
+set -l foo
+show "$foo[1]"
+show $foo[1]
+show "$foo[-1]"
+show $foo[-1]
+show "$foo[2]"
+show $foo[2]
+show "$foo[1 2]"
+show $foo[1 2]
+show "$foo[2 1]"
+show $foo[2 1]