Don't produce invalid XML with multi-line commenting style

Don't produce invalid XML with multi-line commenting style

Both XML and HTML forbid double hyphens inside comments.  However,
nxml-mode was using a `!--' as a comment padding if `comment-style'
was set to any of the styles that supposed to add padding.  This infix
was auto-derived due to `comment-continue' being nil.  To fix that set
`comment-continue' explicitly.  It's unclear what padding should be
used, but from looking at other editors it seems they don't typically
add padding in XML, so let's be simple for now and just set
`comment-continue' to empty string.

* lisp/nxml/nxml-mode.el (nxml-mode): Make 'comment-continue' a
buffer-local variable set to the empty string.  (Bug#71772)