GUACAMOLE-422: Replace non-short-circuit logic in null check (&) with proper short-circuit logic (&&).

Checking `timezone != null & !timezone.isEmpty()` will result in a
`NullPointerException` when `timezone` is null, as the `&` ensures
`timezone.isEmpty()` will run in all cases.

guacamole/src/main/java/org/apache/guacamole/tunnel/TunnelRequestService.java

@@ -167,9 +167,9 @@ public class TunnelRequestService {
         if (imageMimetypes != null)
             info.getImageMimetypes().addAll(imageMimetypes);
         
-        // Get the timezone value
+        // Set timezone if provided
         String timezone = request.getTimezone();
-        if (timezone != null & !timezone.isEmpty())
+        if (timezone != null && !timezone.isEmpty())
             info.setTimezone(timezone);
 
         return info;