fix schema constraint() and tables() if null condition

pom.xml

...in/kotlin/unibz.cs.semint.kprime/domain/ddl/Constraint.kt

src/main/kotlin/unibz.cs.semint.kprime/domain/ddl/Schema.kt

...ain/kotlin/unibz.cs.semint.kprime/domain/dml/ChangeSet.kt

....cs.semint.kprime/usecase/common/ApplyChangeSetUseCase.kt

src/test/kotlin/unibz.cs.semint.kprime/domain/SchemaTest.kt

...ibz.cs.semint.kprime/usecase/ApplyChangeSetUseCaseTest.kt