Some links to external manuals not followable in Info output

[Gavin Smith]

If a cross-reference appears like

@xref{,,, texi2any_api, GNU Texinfo @command{texi2any} Output Customization}.

with no first argument, then this is output in Info like

*Note (texi2any_api)::.

- with no node argument following the manual.  The Info browser (at least
recent versions of it; I checked 6.8 and 7.1) cannot follow such links
(I didn't check with Emacs).  However, texi2any does not warn about
such constructs.

The Texinfo manual condones omitting the first argument:

     So, with cross-references to other manuals (*note Four and Five
  Arguments::), if the first argument is either ‘Top’ (capitalized just
  that way) or omitted entirely, and the third argument is omitted, the
  printed output includes no node or section name.  (The Info output
  includes ‘Top’ if it was given.)

(Info node (texinfo)Referring to a Manual as a Whole.)

I suggest that texi2any should supply the Top node name for links to
external manuals when it is left blank.