BiCreater for Strings lower bound and upper bound should always return an emtpy optional.

dmnverifierapi/src/main/java/de/unikoblenz/fgbks/base/utils/boundary/bicreater/BiCreaterStringLowerBounds.java

@@ -22,6 +22,8 @@ public class BiCreaterStringLowerBounds extends AbstractBoundaryBiCreater<String
 
   @Override
   public Optional<StringBoundary> create(StringBoundary b1, StringBoundary b2) {
+    // Not supported for String Boundary
+    // Check BiCreaterStringBetween.
     return Optional.empty();
   }
 }

dmnverifierapi/src/main/java/de/unikoblenz/fgbks/base/utils/boundary/bicreater/BiCreaterStringUpperBounds.java

@@ -22,6 +22,8 @@ public class BiCreaterStringUpperBounds extends AbstractBoundaryBiCreater<String
 
   @Override
   public Optional<StringBoundary> create(StringBoundary b1, StringBoundary b2) {
+    // Not supported for String Boundary
+    // Check BiCreaterStringBetween.
     return Optional.empty();
   }
 }