Fix logic ensuring parent presence

When we are asked to create intermediate nodes for a path, whose
parent is the conceptual root (i.e. its parent is empty), we should
not attempt to merge it, as it just does not make sense.

JIRA: MDSAL-534
Change-Id: Ia5f6ef2857903d8e88030f4c1f3867b8ed8f5a0e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit ed271c6322092bccf8ff9500b1bbb0fb62683419)