Making the external link icon have a blank alt tag. This icon serves no semantic purpose and it adds a useless (external link) in copy-pasted text from the rendered wiki to another document (such as email or a text editor)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@48180 b456876b-0849-0410-b77d-98878d47e9d5