Allow empty-element tag for base attributes (#679)
Allow empty-element tag for base attributes (#679)

* Allow empty-element tag for base attributes 

Allow empty-element tags for base attributes in node XMLs.

Some tools create node XML with empty elements like *<Description/>*.

open62541 should be able to handle this cause they are valid XML documents

example:
```
<UAReferenceType NodeId="ns=1;i=4001" BrowseName="1:providesInputTo">
        <DisplayName>providesInputTo</DisplayName>
        <References>
            <Reference ReferenceType="HasSubtype" IsForward="false">i=33</Reference>
        </References>
        <InverseName/>
    </UAReferenceType>
```

* Fix #679

AGIsmail/open62541,StalderT/open62541,StalderT/open62541,jpfr/open62541,JGrothoff/open62541,open62541/open62541,jpfr/open62541,jpfr/open62541,StalderT/open62541,AGIsmail/open62541,StalderT/open62541,JGrothoff/open62541,jpfr/open62541,bostjanv/open62541,open62541/open62541,bostjanv/open62541,JGrothoff/open62541,open62541/open62541,JGrothoff/open62541,open62541/open62541