8239585: JFR: Native events should support empty payloads

Reviewed-by: mgronlun

make/src/classes/build/tools/jfr/GenerateJfrFiles.java

@@ -636,6 +636,12 @@ public class GenerateJfrFiles {
             }
             out.write("  }");
         }
+
+        // Avoid clash with static commit() method
+        if (event.fields.isEmpty()) {
+            return;
+        }
+
         out.write("");
         StringJoiner sj = new StringJoiner(",\n                     ");
         if (event.startTime) {

src/hotspot/share/jfr/metadata/metadata.xsd

@@ -45,7 +45,7 @@
           <xs:element maxOccurs="unbounded" name="Event">
             <xs:complexType>
               <xs:sequence>
-                <xs:element maxOccurs="unbounded" name="Field">
+                <xs:element minOccurs="0" maxOccurs="unbounded" name="Field">
                   <xs:complexType>
                     <xs:attribute name="type" type="xs:NMTOKEN" use="required" />
                     <xs:attribute name="struct" type="xs:boolean" use="optional" />