Accessibility: ALT for blank images with " " or ""?

In Brazil, most recommend the use of "spaced" ALT for images completely
meaningless. As all over the world, the recommendation is quite different,
I'd like to check w/ you what's your tech opinion on this one.

I think that "aural" browsers will read the spaced ALT as a pause, a comma.
The null ALT, on the other hand, will be simply ignored, as it should be for
images conveying no information at all. Am I right? Furthermore, is there a
standard on this issue, like a w3 guideline?

Thanks in advance,
Murilo