Handle bogus entries in MediaWiki:Scribunto-doc-page-name

If someone puts something broken in MediaWiki:Scribunto-doc-page-name,
it may not be possible to determine which module the page is a doc page
for. In that case, just pretend it's not a doc page.

Bug: 49322
Change-Id: Ia4421576b372e188cf7e3dfe2c0b8ce213d026d4