fs_mgr: fix SIGSEGV when ostream << nullptr

fstab_rec.fs_options might be nullptr when printing error message.
Use android::base::StringPrintf() to '(null)' when needed.

Bug: 37759782
Test: Boot device and manaully trigger the output
Change-Id: I1bdf4ba57331aaea9dd5e790f6bf9d9b8bdc8b53
Merged-In: I1bdf4ba57331aaea9dd5e790f6bf9d9b8bdc8b53
(cherry picked from commit 59af33c9)