mounts: allow empty returns from service

Allow mount services to return None, which means they have not 
actually mounted anything within the mount root. This might be
because they have bind mounted directories within the tree.
These mounts do not need any path translation.

osbuild/mounts.py

@@ -74,6 +74,11 @@ class MountManager:
         client = mgr.start(f"mount/{mount.name}", mount.info.path)
         path = client.call("mount", args)
 
+        if not path:
+            res = {}
+            self.mounts[mount.name] = res
+            return res
+
         if not path.startswith(self.root):
             raise RuntimeError(f"returned path '{path}' has wrong prefix")