twister: program get stuck when serial in hardware map is empty string

[gopiotr]

Describe the bug
When in hardware map serial option is set as empty string, the program do not rise any error - instead of this it goes forward and gets stuck into wait for device loop

To Reproduce
Steps to reproduce the behavior:

1. Create hardware map file with serial option as empty string. For example like this:

- connected: true
  id: 0006
  platform: nrf52840dk_nrf52840
  product: J-Link
  runner: nrfjprog
  serial: ""

2. Run Twister by command:
   scripts/twister --device-testing --hardware-map map.yml -T samples/hello_world/ -v -v
3. Observe that program stuck into wait for device available loop:

DEBUG - Waiting for device nrf52840dk_nrf52840 to become available
DEBUG - Waiting for device nrf52840dk_nrf52840 to become available
DEBUG - Waiting for device nrf52840dk_nrf52840 to become available

Expected behavior
Program should rise en error (for not existing serial port).

Impact
Annoyance - program get stuck into wait loop and block tests performing.

Environment (please complete the following information):

• Linux
• Zephyr SDK
• Commit SHA: 903c78c

Additional context
I already have proposition for fix this bug and I will create PR soon

[github-actions]

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