No need to partition nodes for the root node

The call to the `partition_nodes/3` function is not needed when
the node it should be partitioned for is the root node, as then
result will always be the input ndoes wrapped in a list.

Change-Id: Ic1b6e7029b8bb095efa1b082283b4a80add6aafc
Reviewed-on: http://review.couchbase.org/40139
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>