YARN-10612. Fix findbugs issue introduced in YARN-10585. Contributed by Gergely Pollak

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/placement/converter/LegacyMappingRuleToJson.java

@@ -138,10 +138,6 @@ public class LegacyMappingRuleToJson {
    * @return JSON Format of the provided mapping rules, null if no rules are set
    */
   public String convert() {
-    if (userGroupMappingRules == null && applicationNameMappingRules == null) {
-      return null;
-    }
-
     //creating the basic JSON config structure
     ObjectNode rootNode = objectMapper.createObjectNode();
     ArrayNode rulesNode = objectMapper.createArrayNode();