redfish: Return empty DBus EventLog collection

When the DBus-backed EventLog entry collection is unavailable, return
an empty Redfish collection instead of an internal error for expected
absence cases.

Treat EBADR and host_unreachable from GetManagedObjects as optional
backend unavailability. In those cases, populate Members with an
empty array and set Members@odata.count to 0, while continuing to
surface unexpected DBus errors as internal errors.

Tested:
- ninja -C build bmcweb

Change-Id: Iffff05231927f8d2239b25bbbb98960d7b5d71e0
Signed-off-by: Joel Pullokaran Jesin <joelpj@ami.com>