scripts: twisterlib: Fix warning about deprecation of truthiness

Python 3.12 warns that

> Testing an element's truth value will raise an exception in future
> versions. Use specific 'len(elem)' or 'elem is not None' test instead.
>    if elem_ts := root.find('testsuite'):

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>