fix: Handle unset AXIS_DEVICE_IP in remote-test.sh (#166)

The previous implementation would fail if `AXIS_DEVICE_IP` was unset
thanks to the `set -u`.

`bin/remote-test.sh`:
- Treat an empty and an unset variable the same because prefixing a
  command with `AXIS_DEVICE_IP= `is a convenient way to temporarily
  force the second branch in the runner.

bin/remote-test.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -eu
 
-if [ -n "${AXIS_DEVICE_IP}" ]; then
+if [ ${AXIS_DEVICE_IP:-} ]; then
     LOCAL_PATH="$1"
     shift