Make nxml-newline-and-indent argument optional

* lisp/nxml/nxml-mode.el (nxml-newline-and-indent): Make argument
optional to conform to 'comment-line-break-function'.  (Bug#40193)

lisp/nxml/nxml-mode.el

@@ -2230,7 +2230,7 @@ ENDP is t in the former case, nil in the latter."
     (skip-line-prefix fill-prefix)
     fill-prefix))
 
-(defun nxml-newline-and-indent (soft)
+(defun nxml-newline-and-indent (&optional soft)
   (delete-horizontal-space)
   (if soft (insert-and-inherit ?\n) (newline 1))
   (nxml-indent-line))