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Perfect example of validation mattering more than WHY it should validate...
Adding alt="" may validate your page, but why does the validator check this? Because an empty alt attribute is of more use than a missing one? No, because images should have (relevant) alt attributes in case the image is broken or the visitor has turned off images.
So, basically, you CAN do what you suggest (shouldn't break anything) - but why not put in a meaningful alt attribute when you are editing the file anyway?
"When you can flatten entire cities at a whim, a tendency towards quiet reflection and seeing-things-from-the-other-fellow's-point-of-view is seldom necessary."
Cusix Software