Fix IllegalArgumentException on Path Endpoint

Steps to reproduce:
Given nuxeo-showcase-content is installed:

• Navigate to localhost:8080/nuxeo/playground and go to Resources Endpoints.
• Under the Path Endpoint, select GET /api/v1/path/{docPath}.
• From the docPath field, browse to the Lorem Ipsum note. (/default-domain/workspaces/Sample Content/Notes/Lorem ipsum note).
• Click on RUN.
  => Response is:

  {
    "entity-type": "exception",
    "status": 500,
    "message": "java.lang.IllegalArgumentException: Illegal path with empty component: //default-domain/workspaces/Sample Content/Notes/Lorem ipsum note"
  }
  

Workaround:

• After selecting the path, remove the first / and click on RUN.