Fix an issue where editing paths in Owners packages could raise an error: undefined index "display"

Summary:
Fixes T13464. Fully-realized paths have a "path" (normalized, effective path) and a "display" path (user-facing, un-normalized path).

During transaction validation we build ref keys for paths before we normalize the "display" values. A few different approaches could be taken to resolve this, but just default the "display" path to the raw "path" if it isn't present since that seems simplest.

Test Plan: Edited paths in an Owners package, no longer saw a warning in the logs.

Maniphest Tasks: T13464

Differential Revision: https://secure.phabricator.com/D20923