EZP-22062 Fixed error 500 on /api/ezp/v2/content/locations/1

The problem here was that when we call curl/api/ezp/v2/content/locations/1 we get an error 500 (internal server error). This comes from the fact that 'ezpublish_rest_loadLocation' should match [0-9/]+ but here root node location doesn't have any parent location attribute so ezpublish_rest_loadLocation could not match.

- [x] Fixed error 500 when call curl/api/v2/content/locations/1
- [x] Added unit tests



Jira link : https://jira.ez.no/browse/EZP-22062