check for null queryString in handleSignatureRequest()

check for null queryString in handleSignatureRequest() before performing contains() check. In Tomcat 8, this value is null when the query string is empty (Ah Java how you love the NPE's...)

java/s3/S3Uploads.java

@@ -93,7 +93,7 @@ private void handleSignatureRequest(HttpServletRequest req, HttpServletResponse
         JsonElement contentJson = jsonParser.parse(req.getReader());
         JsonObject jsonObject = contentJson.getAsJsonObject();
 
-        if (req.getQueryString().contains("v4=true")) {
+        if (req.getQueryString() != null && req.getQueryString().contains("v4=true")) {
             handleV4SignatureRequest(jsonObject, contentJson, req, resp);
         }
         else {