It appears that searches with single character queries are redirected to the site's search help page. For example:
This is acceptable, because with the English language there are few valid search terms that are only one characters long. However, for newer sites such as Chinese Language and Usage, a single Unicode character can be equivalent to one or more English words. I believe therefore that the restriction should be relaxed for these sites. For instance, a search like this:
is valid, and should work.