Allow local interwiki links with an empty title part

Allow local interwiki links with an empty title part

For example, [[en:]] on enwiki. These links were for some reason considered
invalid. However, to support use-cases such as cross-wiki transclusion and
the importing of templates across wikis, this type of link should be
allowed.

I can't see that this would break anything. If anything, it will "un-break"
some links.

Should MediaWikiTitleCodec depend on Title::newMainPage? Almost certainly
not, but I note that splitTitleString is earmarked for demolition, so I
expect that the confusing dependency-web will be cleaned up in the future.
I also note that MediaWikiTitleCode already uses various static methods of
the Title class.

Bug: 64167
Change-Id: I27495aa62e6059c7725387135ae0358d7e6fb35b