inline: When the pagenames list includes pages that do not exist, skip them.

bestlink returns '' if no existing page matches a link. This propigated
through inline and other plugins, causing uninitialized value warnings, and
in some cases (when filecheck was enabled) making the whole directive fail.

Skipping the empty results fixes that, but this is papering over another
problem: If the missing page is later added, there is not dependency
information to know that the inline needs to be updated. Perhaps smcv will
fix that later.