Check null

See https://github.com/strongloop/loopback-datasource-juggler/issues/223
Bump version
Merge branch 'release/2.4.1' into production
Merge tag 'v2.4.1'

2.4.1
Properly handle LDL for polymorphic relations
Merge branch 'fix/ldl-polymorphic' of github.com:fabien/loopback-datasource-juggler into fix/ldl-polymorphic