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.

0 files changed, 0 insertions, 0 deletions