mgr: Don't log null characters in PerfCounter paths

OSD logs at debug level 20 can contain 0x0's caused
by MgrClient logging PerfCounter path strings which
intentionally include 0x0 characters.

Filter these strings to change these nulls to spaces.

Fixes: https://tracker.ceph.com/issues/78642

Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>