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.
Double expansions of variables had the following issues:
* `"$$foo"` threw an error no matter what the value of `$foo` was.
* `set -l foo ''; echo $$foo` threw an error because of the expansion of
`$foo` to `''`.
With this change, double expansion always works properly. When
double-expanding a multi-valued variable, in a double-quoted string the
first word of the inner expansion is used for the outer expansion, and
outside of a quoted string every word is used for the double-expansion
in each of the arguments.
> set -l foo bar baz
> set -l bar one two
> set -l baz three four
> echo "$$foo"
one two baz
> echo $$foo
one two three four
