Section-index: ensure .Parent isn't nil

[chalin]

• Follow up to Fix warning: '.File.UniqueID on zero object' (#1007) #1633
• Fixes 0.9.0: execute of template failed at <.Parent.File>: nil pointer evaluating page.Page.File -- error when using page type docs #1863

To test, I ran the following from a local docsy repo:

$ mkdir tmp
$ cd tmp
$ ../tools/make-site.sh -s HUGO_MODULE
$ cd test-site
$ echo "---\ntitle: Hello\ntype: docs\n---\nPlaceholder" > content/_index.md
$ hugo # building or serving seems fine now

Without the fix in this PR, the commands above yield the error reported in #1863.

[chalin]

/cc @marshalc @chulcher

[chulcher]

The change to 'if and .Parent .Parent.File' is working for my setup.

[deining]

LGTM.
Sorry for introducing a regression here.