lisa.wa: Fix common prefix for wa_path

Created by: vdonnefort

When filling wa_path, WAOutput is looking for the longest common path, shared by all the WA folder path. e.g

foo/bar foo/bar2 would become respectively bar and bar2.

A problem occurs when using a path with WAOutput that is the root of a WA folder. (e.g. foobar where foobar/__meta exists). This common path (common_prefix) would then be PosixPath('.'). Pathlib cannot use a such path with relative_to() and raises the following error:

ValueError: '/foo/bar/' does not start with ''

Fixing this problem by converting common_prefix to an absolute path.