If the body of a man(7) .MT or .UR block is empty, do not emit a warning.

If the body of a man(7) .MT or .UR block is empty, do not emit a warning.
Leaving the body empty is legitimate in this case if the author only
wants to display a mail address or URI without providing a link text.
Output modules already handle this correctly: terminal output shows
just the URI without an accompanying text, HTML output uses the URI
for *both* the href= attribute and as the content of the <a> element.

The documentation was also wrong and claimed that an .MT or .UR block
with an empty body would produce no output.  As explained above,
this isn't true.

Bogus warning reported by
Alejandro Colomar <alx dot manpages at gmail dot com>.