set null for null headers, instead of ignoring them

Author: kashyap-splunk

src/main/java/com/splunk/kafka/connect/SplunkSinkTask.java

@@ -461,8 +461,12 @@ private Event addHeaders(Event event, SinkRecord record) {
             Map<String, String> headerMap = new HashMap<>();
             for (String header : customHeaders) {
                     Header customHeader = headers.lastWithName(header);
-                if (customHeader != null && customHeader.value() != null) {
-                    headerMap.put(header, customHeader.value().toString());
+                if (customHeader != null) {
+                    if (customHeader.value() == null) {
+                        headerMap.put(header, null);
+                    } else {
+                        headerMap.put(header, customHeader.value().toString());
+                    }
                 }
             }
             event.addFields(headerMap);