Fix `Target` type validation constraint

[alexander-yevsyukov]

This PR adds (is_required) validation constraint to the criterion group of the Target type. Previously it was assumed, now it's explicit.

It also extends EntityQueryProcessor to force validation of the query.target until Validation library does it in the generated code.

Other problems related to validations were also fixed.

[codecov[bot]]

Codecov Report

Merging #1482 (0c62996) into master (069f5c4) will decrease coverage by 0.01%. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1482 +/- ## ============================================ - Coverage 91.74% 91.72% -0.02% + Complexity 4971 4968 -3 ============================================ Files 627 627 Lines 15234 15235 +1 Branches 872 873 +1 ============================================ - Hits 13977 13975 -2 - Misses 953 956 +3 Partials 304 304 ```