load_by_path() should check if caching is enabled

This is something of a follow up to #1941968: Check for cached data fails.

In path_breadcrumbs_load_by_path(), there is an assumption that caching is enabled. So, PB loads data from the cache, and a new breadcrumb I added never gets picked up.

A very simple fix is to check if caching is enabled before attempting to load data from the cache.

Patch is coming soon.