Properly allow searches by name without UUID

src/main/java/com/gmail/nossr50/database/SQLDatabaseManager.java

@@ -1587,7 +1587,7 @@ public final class SQLDatabaseManager implements DatabaseManager {
 
         try {
             statement = connection.prepareStatement("SELECT id, user FROM " + tablePrefix + "users WHERE uuid = ? OR (uuid IS NULL AND user = ?)");
-            statement.setString(1, uuid.toString());
+            statement.setString(1, uuid == null ? null : uuid.toString());
             statement.setString(2, playerName);
             resultSet = statement.executeQuery();