redis-brain: Check if info.path is null before replacing strings

Fixes the non-prefix case broken by #1383.

Until this lands, you can work around the error (discussed in
fdf19a3):

  ERROR Unable to load /.../redis-brain: TypeError: Cannot call method 'replace' of null

by ensuring REDIS_URL includes a trailing slash
(e.g. 'redis://localhost:6379/' instead of 'redis://localhost:6379').