Do not create log dir when the results_dir is null

The results_dir could be null is test_runner_handler.get_test_runner_reqs(),
and results in generating $PWD/log whenever running atest command. This
change fix the bug that it will only create log dir when results_dir is
given.

Bug: 275537997
Test: atest-dev hello_world_test --host # ensure no log dir in $PWD.
Change-Id: I4e4fbb61845bfe6173a2476e031c2858bd3b48e2
1 file changed