xrootd: allow empty xrootd.authz.read-paths and xrootd.authz.write-paths

The default value for xrootd.authz.write-paths is the empty string, but
the "check -strong" prevents this default value from working.

This patch removes the strong check for both paths.

Target: master
Requires-book: no
Requires-notes: no
Acked-by: Gerd Behrmann
Patch: https://rb.dcache.org/r/7272/