update null check

Author: deleonenriqueta

logback13/src/main/java/com/newrelic/logging/logback13/NewRelicAsyncAppender.java

@@ -62,7 +62,9 @@ protected void append(ILoggingEvent eventObject) {
         Map<String, String> copyMdc = MDC.getMDCAdapter().getCopyOfContextMap();
         if (copyMdc != null) {
             for (Map.Entry<String, String> entry : copyMdc.entrySet()) {
-                combinedContextMap.put(CONTEXT_PREFIX + entry.getKey(), entry.getValue());
+                if (entry.getValue() != null) {
+                    combinedContextMap.put(CONTEXT_PREFIX + entry.getKey(), entry.getValue());
+                }
             }
         }
 

logback13/src/main/java/com/newrelic/logging/logback13/NewRelicJsonLayout.java

@@ -74,6 +74,9 @@ private void writeToGenerator(ILoggingEvent eventObject, JsonGenerator generator
             String value;
 
             for (Map.Entry<String, String> entry : mdcPropertyMap.entrySet()) {
+                if (entry.getValue() == null) {
+                    continue;
+                }
                 key = entry.getKey();
                 value = entry.getValue();
                 generator.writeStringField(key, value);