Check if Authorization header is empty

This resolves s3-tests failures that we missed due to busted CI.
Perhaps caused by a newer boto version?

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -369,7 +369,7 @@ public class S3ProxyHandler {
         boolean presignedUrl = false;
 
         if (!anonymousIdentity) {
-            if (headerAuthorization == null) {
+            if (Strings.isNullOrEmpty(headerAuthorization)) {
                 String algorithm = request.getParameter("X-Amz-Algorithm");
                 if (algorithm == null) { //v2 query
                     String identity = request.getParameter("AWSAccessKeyId");