This issue has been confused with Bug 32189 and requires a separate bug. The use of a link to a wikisource language subdomain that does not exist should redirect to the multilingual wikisource via missing.php (http://svn.wikimedia.org/viewvc/mediawiki/trunk/tools/web-scripts/missing.php?view=markup).
In the past new language subdomains have asked for direct interwiki links (see eo.ws (Bug 17008) and be.ws (Bug 26124)). Although creating aliases for language subdomains in active incubation may be a good idea, this bug is intended to deal with links created to valid languages irrespective of whether works exist.
The point of this bug is so that, for example, on hi.wp [[s:Foo]] would link to hi.wikisource.org/wiki/Foo which would redirect to wikisource.org/wiki/Foo and on en.ws [[hi:Foo]] would create a sidebar link to the same page. AND the same would happen for any other valid language code without a subdomain regardless of whether any works actually existed.