Prevent potential NPE while closing resource

This prevents the `java.lang.NullPointerException: Cannot invoke "io.quarkus.bootstrap.runner.JarFileReference.close(io.quarkus.bootstrap.runner.JarResource)" because "ref" is null` error

independent-projects/bootstrap/runner/src/main/java/io/quarkus/bootstrap/runner/JarResource.java

@@ -154,8 +154,9 @@ public void close() {
             // The jarfile has been already used and it's going to be removed from the cache,
             // so the future must be already completed
             var ref = futureRef.getNow(null);
-            assert (ref != null);
-            ref.close(this);
+            if (ref != null) {
+                ref.close(this);
+            }
         }
     }