OpenaApi: Use empty content for operational GET

OpenaApi: Use empty content for operational GET

Previously, operational data was included in schemas
to provide a GET example for operational containers and lists.
However, removing this operational data caused an error in
the OpenAPI UI since the expected data was not found.

To address this issue, the operational data can be replaced
with empty content instead. Creating a duplicate schema
specifically for this use case is both memory-intensive
and unnecessary.

This commit solves the problem by adding empty example
for the operational GET data.

JIRA: NETCONF-1061
Change-Id: I1444aad588c42417e5ff21dee46ff91ec0a1a07d
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>