Fix html-scanner crash in SanitizeHelper#sanitize for empty protocols

As detailed in issue #8126 there is a bug in the vendored html-scanner gem that makes WhiteListSanitizer#contains_bad_protocols? crash when ```<a href=":"></a>``` is passed to it.

This bugfix is a simple fix in the regular expression of #contains_bad_protocols? in order to only match protocols with at least one character - so that the code does not throw "undefined method `downcase' for nil:NilClass".

We have encountered this kind of HTML in user-generated content, and even though I agree its pretty weird to pass something like that in, there is no other way to avoid this problem for us whilst still sanitizing other tags in the same input.