drivers: i2c_nrfx_twim: Fix handling of zero-length transfers

For a zero-length transfer, the STOP task is not triggered
automatically by the shortcut with the event that signals
the transfer end because such event is not generated.
Trigger this task "manually" to prevent the driver getting
stuck after the address byte is acknowledged.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>