WW-5546 Fixes NPE when uploaded file is empty (#1263)

Author: lukaszlenart

core/src/main/java/org/apache/struts2/interceptor/AbstractFileUploadInterceptor.java

@@ -115,7 +115,7 @@ protected boolean acceptFile(Object action, UploadedFile file, String originalFi
         }
 
         // If it's null the upload failed
-        if (file == null) {
+        if (file == null || file.getContent() == null) {
             String errMsg = getTextMessage(action, STRUTS_MESSAGES_ERROR_UPLOADING_KEY, new String[]{inputName});
             if (validation != null) {
                 validation.addFieldError(inputName, errMsg);