check whether the network interface exists before using it

Some build targets of the cuttlefish project don't have the
ethernet interface. This CL checks whether the network interface
exists before setting it as Thread simulation local interface.

Bug: b/378777255
Test: acloudw create --branch git_main --build-id P85047583 --build-target cf_x86_64_auto-trunk_staging-userdebug
Change-Id: Ifb1e316f59fac7babf34a7c290d5234853de8653