Fixed calling DisplayViewer or XVViewer without a title

[radarhere]

Requested in 8da8013#r68650261
Resolves #6146

That comment points out that

Pillow/src/PIL/ImageShow.py

Lines 51 to 61 in 6faebd3

	def show(image, title=None, **options):
	r"""
	Display a given image.
	:param image: An image object.
	:param title: Optional title. Not all viewers can display the title.
	:param \**options: Additional viewer options.
	:returns: ``True`` if a suitable viewer was found, ``False`` otherwise.
	"""
	for viewer in _viewers:
	if viewer.show(image, title=title, **options):

can set title to None, which is then passed (through Viewer.show and show_image) to DisplayViewer and XVViewer as None for one of the values passed to subprocess.Popen.

This problem was introduced in #6010.