tiny-test-fw: handle case when ESPPORT is not set
tiny-test-fw: handle case when ESPPORT is not set

Fix bug introduced in 8a88dd35, where .decode was called on result of
os.getenv, which was None if ESPPORT was not set.

espressif/esp-idf,mashaoze/esp-idf,mashaoze/esp-idf,mashaoze/esp-idf,www220/esp-idf,armada-ai/esp-idf,espressif/esp-idf,armada-ai/esp-idf,armada-ai/esp-idf,www220/esp-idf,mashaoze/esp-idf,www220/esp-idf,espressif/esp-idf,espressif/esp-idf,mashaoze/esp-idf,armada-ai/esp-idf,www220/esp-idf,www220/esp-idf