fix: report empty projects as not locked

GET /projects/{id}/locked returned True for a project with no drawings
or nodes: both loops ran zero times and the fallback return won. Locking
and unlocking such a project always succeeded while the state stayed
"locked", so it could never be unlocked.

Report a project with nothing to lock as not locked, and re-check the
state after unlock in the route tests.

gns3server/controller/project.py

@@ -2141,6 +2141,10 @@ class Project:
         Check if all items in a project are locked and not
         """
 
+        if not self._drawings and not self._nodes:
+            # a project without drawings or nodes has nothing to lock and would
+            # otherwise always report as locked, even after unlocking it
+            return False
         for drawing in self._drawings.values():
             if not drawing.locked:
                 return False