analysis.xml: fix potential null dereference

analysis.xml: fix potential null dereference

File#listFiles() can return null. Even though the test
"isDirectory()" and "exists()" are able to pick it up, executing
the command and null checking it is the best garanty.

Change-Id: Ic3cefdaecf3d5d88dbeaeb3f3d58689abdb05347
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/65332
Reviewed-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>