[HUDI-1892] Fix NPE when avro field value is null (#3051)

hudi-common/src/main/java/org/apache/hudi/common/model/OverwriteWithLatestAvroPayload.java

@@ -94,6 +94,6 @@ public class OverwriteWithLatestAvroPayload extends BaseAvroPayload
    * Return true if value equals defaultValue otherwise false.
    */
   public Boolean overwriteField(Object value, Object defaultValue) {
-    return defaultValue == null ? value == null : defaultValue.toString().equals(value.toString());
+    return defaultValue == null ? value == null : defaultValue.toString().equals(String.valueOf(value));
   }
 }