Improve type checking for percent

It's unlikely a user would ever want to pass 0, but to remain true to
what's passed in, allow this.

Author: tony

libtmux/window.py

@@ -454,7 +454,7 @@ def split_window(
         else:
             tmux_args += ('-h',)
 
-        if percent:
+        if percent is not None:
             tmux_args += ('-p %d' % percent,)
 
         tmux_args += ('-P', '-F%s' % ''.join(tmux_formats))  # output