mon: let CrushWrapper::get_validated_type_id() return an optional<>

mon: let CrushWrapper::get_validated_type_id() return an optional<>

easier to reason about. and it helps to silence the false alarm of

../src/mon/OSDMonitor.cc: In member function 'void OSDMonitor::try_enable_stretch_mode(std::stringstream&, bool*, int*, bool, const string&, uint32_t, const std::set<pg_pool_t*>&, const string&)':
../src/mon/OSDMonitor.cc:14379:44: warning: 'dividing_id' may be used uninitialized in this function [-Wmaybe-uninitialized]
14379 |       pool->peering_crush_bucket_barrier = dividing_id;
      |                                            ^~~~~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>