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, 4 insertions, 0 deletions

diff --git a/tests/expansion.out b/tests/expansion.out
index 3e763180..fe594539 100644
--- a/tests/expansion.out
+++ b/tests/expansion.out
@@ -36,3 +36,7 @@
 2 baz quux
 1 prefixbaz quux  fooest
 2 prefixbaz prefixquux
+1 
+0
+1 
+0