NullPointerException when entering data with undefined element at first level

Tested on: stable/beryllium
Caught by: MDSAL Northbound suite (https://git.opendaylight.org/gerrit/33643/)

1. Make sure odl-clustering-test-app is NOT installed.
2. Open a Netconf connection to ODL and send the following request:

<rpc message-id="2" a="64" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<candidate/>
</target>
<default-operation>merge</default-operation>
<config>
<cars xmlns="urn:opendaylight:params:xml:ns:yang:controller:config:sal-clustering-it:car">
<car-entry>
<id>TOY001</id>
<model>Toyota Corolla</model>
<manufacturer>Toyota</manufacturer>
<year>2016</year>
<category>sedan</category>
</car-entry>
</cars>
<people xmlns="urn:opendaylight:params:xml:ns:yang:controller:config:sal-clustering-it:people">
<person>
<id>CUST001</id>
<age>32</age>
</person>
</people>
<car-people xmlns="urn:opendaylight:params:xml:ns:yang:controller:config:sal-clustering-it:car-people">
<car-person>
<car-id>TOY001</car-id>
<person-id>CUST001</person-id>
</car-person>
</car-people>
</config>
</edit-config>
</rpc>

3. You will get the following response:

<rpc-reply a="64" message-id="2" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<rpc-error>
<error-type>application</error-type>
<error-tag>operation-failed</error-tag>
<error-severity>error</error-severity>
<error-message>Unexpected error</error-message>
<error-info>
<error>java.lang.NullPointerException</error>
</error-info>
</rpc-error>
</rpc-reply>

I expected something along the lines of "Element 'cars' not found" or "No model for element 'cars' found" or even "Unknown child element(s) ... 'cars' ...". Or something mentioning "people" or "car-people" or even their namespaces (urn:opendaylight:params:xml:ns:yang:controller:config:sal-clustering-it:

).