I have tested using the latest version Q2 2013 and it seems to work the same for RadTextBox and Asp:TextBox.
Indeed the " " is threaded as null, since the code of ASP Validator is like follows:
m = s.match(/^\s*(\S+(\s+\S+)*)\s*$/);
(ValidatorTrim(ValidatorGetValue(val.controltovalidate)) != ValidatorTrim(val.initialvalue))
This means that string that does not contain characters different than space will considered as null.
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