Fix for indexes on custom object types that do not resolve to any AttributeType but implement Comparable.
Key change is at IndexImpl.saveEntryIndex (lines 80-85). Even if attribute type is null we can still use an index.
Test: QueryIndexTest.testResultsReturned_whenCustomAttributeIndexed
Also, split up QueryBasicTest into multiple files.
Merge pull request #4053 from ajermakovics/mast/unknown-type-indexes

Support for indexes on custom attribute types