Allows inputting of empty lines.

Empty lines ("") are currently not being parsed by Evennia and the CMD_NOINPUT command handler won't run for them. Adding a space (" ") makes the command parseable by Evennia.

This pull request fixes this so that an empty string will also run the CMD_NOINPUT command.
This is an issue in for instnce the menu_login contrib: an empty lines is used to return back to the previous menu item, but that didn't work.