tree: return tree root from tree[{}].

Fix #41.
* specs/tree_spec.yaml (returns tree root for an empty list): New
specification.
* lib/std/tree.lua (__index): Allow empty list as a valid key.
* specs/tree_spec.yaml (std.tree): Remove pending call for
newly passing examples.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>