Empty lists/maps not supported

TAML allows empty strings but does not allow:
- empty lists
- empty maps

This seems like an odd omission since empty (string) values are supported.

Start by reproducing issue #29 with TAML inputs containing an empty list and an empty map, then trace the parser entry point that handles collection values. Add coverage for both cases and verify that empty strings continue to work while both empty collections are accepted.

Written by the indexing model from the issue text.