[ML] Do not serialize rules when empty (elastic/x-pack-elasticsearch#4203)

Original commit: elastic/x-pack-elasticsearch@18d731cb35

plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/job/config/Detector.java

@@ -292,7 +292,9 @@ public class Detector implements ToXContentObject, Writeable {
         if (excludeFrequent != null) {
             builder.field(EXCLUDE_FREQUENT_FIELD.getPreferredName(), excludeFrequent);
         }
+        if (rules.isEmpty() == false) {
             builder.field(RULES_FIELD.getPreferredName(), rules);
+        }
         // negative means "unknown", which should only happen for a 5.4 job
         if (detectorIndex >= 0
                 // no point writing this to cluster state, as the indexes will get reassigned on reload anyway