8255992: JFR EventWriter does not use first string from StringPool with id 0

Reviewed-by: egahlin

src/jdk.jfr/share/classes/jdk/jfr/internal/StringPool.java

@@ -49,7 +49,7 @@ public final class StringPool {
     }
     private static class SimpleStringIdPool {
         /* string id index */
-        private final AtomicLong sidIdx = new AtomicLong();
+        private final AtomicLong sidIdx = new AtomicLong(1);
         /* epoch of cached strings */
         private boolean poolEpoch;
         /* the cache */