support/run-tests: check for empty sequences in a pythonic way

According to PEP8 empty sequences should be checked as booleans.

Fixes the following PEP8 warning:
Do not use `len(SEQUENCE)` to determine if a sequence is empty

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>