Do not invoke LinkPersister#isLinkContent for attributes

As in this case, we KNOW it's an attribute, and attribute
content is what we are interested in.
Fixing mocked UTs for NPE

As new call is added now for UID
Added test verifying that attribute files are not checked for being a link
Merge pull request #446 from sonatype/nexus-2.8.x-no-link-check-for-attributes

[2.8.x] Do not invoke LinkPersister#isLinkContent for attributes