For what it's worth, W3C HTML Validator claims that a document with an
empty string in the href attribute of a base tag

    <base href="">

is valid. I found such document recently in my INBOX and Gnus was unable
to display it correctly because the second string-match in
shr-parse-base breaks when local is nil. My solution is not to call
shr-parse-base not only when href is not present but also when href is
an empty string.

The patch has been crated on the current master branch.
-- 
Kind regards,
Łukasz Stelmach