CNDB-15558: ULID-based SSTable ID generation can fail with an NPE

Author: michaelsembwever

src/java/org/apache/cassandra/io/sstable/ULIDBasedSSTableId.java

@@ -133,7 +133,7 @@ public Supplier<ULIDBasedSSTableId> generator(Stream<SSTableId> existingIdentifi
                     {
                         newVal = ulid.nextValue();
                     }
-                } while (newVal != null && !prevRef.compareAndSet(prevVal, newVal));
+                } while (newVal == null || !prevRef.compareAndSet(prevVal, newVal));
                 return new ULIDBasedSSTableId(newVal);
             };
         }