Bugfix: Empty rows are not prime in NL*. Fixes Issue 70 (#71)

Co-authored-by: Joshua Moerman <joshua@cs.rwth-aachen.de>

Author: Jaxan

algorithms/active/nlstar/src/main/java/de/learnlib/algorithms/nlstar/Row.java

@@ -129,7 +129,9 @@ public List<Row<I>> getCoveredRows() {
     }
 
     boolean checkPrime() {
-        if (coveredRows.isEmpty()) {
+        if (contents.isEmpty()) {
+            prime = false;
+        } else if (coveredRows.isEmpty()) {
             prime = true;
         } else {
             BitSet aggContents = new BitSet();