Cannot start Sleep GUI without a hypnogram

[Teteros]

The prompt always opens, even with hypno=None passed to visbrain.gui.Sleep()

visbrain/visbrain/io/read_sleep.py

Lines 121 to 126 in b599038

	if hypno is None:
	hypno = dialog_load(self, "Open hypnogram", upath,
	"Text file (*.txt);;Elan (*.hyp);;"
	"CSV file (*.csv);;EDF+ file(*.edf);"
	";All files (*.*)")
	hypno = None if hypno == '' else hypno

doing hypno='' triggers an assertion, while chosing an empty file errors.
So there is no way to start Sleep without exiting the hypnogram open file prompt every time.

Sleep doesn't force the open prompt with config and annotation files, so this seems strange to do.
Adding a boolean value for it or changing the behaviour to be the same as the latter two parameters should be an easy fix I think.