(fix)(chat) Fix the null pointer issue in fieldNameToDateFormat. (#1587)

headless/api/src/main/java/com/tencent/supersonic/headless/api/pojo/SchemaElement.java

@@ -90,6 +90,6 @@ public class SchemaElement implements Serializable {
         if (StringUtils.isNotBlank(timeFormat) && containsPartitionTime()) {
             return timeFormat;
         }
-        return null;
+        return "";
     }
 }