fix the Thread Network fuzz testing error

The current fuzz testing comunicates with the Thread radio chip
simulation program `ot-rcp`. The 'ot-rcp' is not installed by all
devices, which causes the fuzz testing program is unable to find
the program '/vendor/bin/ot-rcp'.

This commit enables the HAL to talk to the '/dev/null' to avoid
'/vendor/bin/ot-rcp' not being found.

Bug: 289627270
Test: Run the fuzz test on cuttlefish
Change-Id: If410d7d75abe05c7c2d2c82fa0450e925b9a48bb