Floss: null-check on transport instance use

This CL guards the transport instance uses (which can come from a remote
process via DBus) so that it does not result in crash when the instance
is null.

Bug: 317682584
Test: m Bluetooth
Flag: EXEMPT floss only changes
Change-Id: I7a08231ba8c8c37941b3fb1294d09de60853d3f7