Additional input parameter check to persist and unpersist function in fs
utils. 

Passing None as path will no longer cause for a crash.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1154356
Change-Id: Icd6a0010e4ffdfd92b83506f1809ff54f5f40e10
Signed-off-by: Tolik Litovsky <tlitovsk@redhat.com>