Add support for converting OptionalPath to Paths

Appending a Path or Paths to a slice of Paths is simple but appending
an OptionalPath requires conditional logic which makes OptionalPaths
harder to use. This change makes it easy to append the embedded Path,
if any, to a slice of Paths.

Bug: 179354495
Test: m nothing
Change-Id: Ibf80a23043c846162e17c3a98b2590bca653b170