[IMP] util/helpers: tests for import/export empty paths

[pauloamed]

Commits 5b944f7cad5512d90938a28b6db93efb9231a5d8 and 1d9e20e5e8838664ccfb025580a2e0f3ec1d535a did not consider the case of empty paths, what was fixed in a05df6682b5d72c8d059872755e75c80d44d32c2. Due to its urgent nature, no tests were added with the fix's commit.

That said, this commit adds tests for the case when base_import.mapping or ir.export.line present path fields which are empty.