Fix regression in `:parent_example_group` metadata.

For a top-level group, it should not be in the metadata,
as it has no parent example group.

As a bonus, this also fixed a couple things I couldn’t get
to work quite right in the backwards compatibility
`:example_group` support.

Fixes #1552.
Merge pull request #1557 from rspec/fix-metadata-regression

Fix regression in `:parent_example_group` metadata.