GUACAMOLE-195: Remove incorrect check on empty username string.

extensions/guacamole-auth-http/src/main/java/org/apache/guacamole/auth/http/AuthenticationProviderService.java

@@ -74,7 +74,7 @@ public class AuthenticationProviderService {
         HttpServletRequest request = credentials.getRequest();
         if (request != null) {
             String username = request.getRemoteUser();
-            if(username == null || username == "") {
+            if(username == null) {
                 username = request.getHeader(confService.getHttpAuthHeader());
             }
             if (username != null) {