Fix incorrect schema reference in root POST link

Fix incorrect schema reference in root POST link

We are creating a root POST link, when requesting swagger document
for single module. We were passing empty string as module name
when building the POST operation, which resulted in our schema ref
beginning with underscore (like this: "_toaster_module").
The schema is created, but without the leading underscore.

This patch fixes it by passing null value and adding logic
that performs the null checking to construct correct schema reference.

JIRA: NETCONF-1133
Change-Id: I9019dea1874133706f8825b0159e6c571d7b9558
Signed-off-by: Šimon Ukuš <simon.ukus@pantheon.tech>