feat: add NE predicate for custom attributes criteria expressions

[igdianov]

This PR adds support for NE predicates on custom attributes criteria expressions, i.e.

query {
  TaskVariables(where: { 
    value: {NE: null} 
  }) {
    select {
      name
      value
    }
  }
}

[codecov]

Codecov Report

Merging #140 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #140      +/-   ##
============================================
+ Coverage     68.23%   68.28%   +0.05%     
- Complexity      437      439       +2     
============================================
  Files            33       33              
  Lines          2238     2242       +4     
  Branches        331      333       +2     
============================================
+ Hits           1527     1531       +4     
  Misses          576      576              
  Partials        135      135

Impacted Files	Coverage Δ	Complexity Δ
...hql/jpa/query/schema/impl/JpaPredicateBuilder.java	52.98% <100%> (+0.76%)	58 <0> (+2)	⬆️

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