MODE-2174 JCR Query empty string in comparison

This is a fix solution of a bug [MODE-2174](https://issues.jboss.org/browse/MODE-2174) "JCR Query in Modeshape returns no result when empty string used in comparison" for ModeShape **3.7.x** version.

This approach is based on using Lucene post processing constraints. Every query is checked for presence of empty string constraint (see class ```LuceneQueryDistributor```) and if so, then push this constraint into post processing.
It's not clear to me why `isQueryContainsEmpty(...)` for `WHERE x IS NOT NULL` simply calls  `isQueryContainsEmpty(...)` for `WHERE x IS NULL`. Why are those equivalent?