Correctly handle passkey of zero

Previously, we treated 0 as "no passkey" / do not display passkey. Now
we don't.

I don't know how this handles the case where the remote device provides
0 and we auto-accept - can this be tested?

Test: manual
Tag: #stability
Bug: 241173545
Change-Id: I992d1ae120b949eec19414b1cd08bdf40797706a