Fix nil exception with docs section list without parent file
While writing documentation in a SSoT, I added a new directory and file in the docs/ directory. Hugo 0.123.7 renders a nil pointer exception with docsy 0.9.1
After investigation, I found an issue https://github.com/google/docsy/issues/1863 and the following fixes
- https://github.com/google/docsy/pull/1866/files
- https://github.com/tobiaskohlbau/docsy/commit/3933271f42cea7fd9143b2aa0accbdc9bd04c9cd
Since we already override the partial for some reason, this MR applies the fixes.