8350771: Fix -Wzero-as-null-pointer-constant warning in nsk/monitoring ThreadController utility

Reviewed-by: dholmes

Author: Kim Barrett

test/hotspot/jtreg/vmTestbase/nsk/monitoring/share/ThreadController.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -465,8 +465,8 @@ extern "C" {
                 return env->NewStringUTF("TIMED_WAITING");
             }
         // should never reach
-        assert(0);
-        return 0;
+        assert(false);
+        return nullptr;
     }
 
     /*