mavsdk_tests: don't forget to check for rootfs dir

test/mavsdk_tests/process_helper.py

@@ -177,6 +177,7 @@ class Px4Runner(Runner):
         rootfs_path = self.cwd
         if self.verbose:
             print("Clearing rootfs (except logs): {}".format(rootfs_path))
+        if os.path.isdir(rootfs_path):
             for item in os.listdir(rootfs_path):
                 if item == 'log':
                     continue