Fix null pointer in window.name

[phil294]

Most X11 windows apparently have no name, and xdotool (done with version 2016, the previous / working one) returns a null pointer. x_do.cr however expects a string. This adds a null check so that window.name returns nil in that case instead of failing with Unhandled exception: Cannot create a string with a null pointer (ArgumentError).

Demo code:

require "x_do"

XDo.act do
    wins = search do
        window_name "" # all (regex)
    end
    puts wins.map &.name
end