fix build failure.

Author: vicheey

aws-serverless-java-container-core/src/main/java/com/amazonaws/serverless/proxy/internal/servlet/AwsHttpServletRequest.java

@@ -517,7 +517,10 @@ protected Map<String, List<Part>> getMultipartFormParametersMap() {
                 newPart.setContentType(item.getContentType());
                 newPart.setSize(item.getSize());
                 item.getHeaders().getHeaderNames().forEachRemaining(h -> {
-                    newPart.addHeader(h, item.getHeaders().getHeader(h));
+                    String headerValue = item.getHeaders().getHeader(h);
+                    if (headerValue != null) {
+                        newPart.addHeader(h, headerValue);
+                    }
                 });
 
                 addPart(multipartFormParameters, item.getFieldName(), newPart);