lisp/ox-latex: Omit empty date

* lisp/ox-latex.el: (org-latex-template): When no date is provided or
the export option date is nil, the empty '\date{}' string is no
longer included in the exported latex document.

The default behaviour for ox-latex was to include an empty date line as
'\date{}'.  This empty date line can affect more complex latex templates
when the date is for example set by the template on compilation, or when
the date command is overwritten entirely.  In this new patch, the date
line is omitted from the exported document.

TINYCHANGE

lisp/ox-latex.el

@@ -1977,7 +1977,8 @@ holding export options."
 	     ((or author email) (format "\\author{%s}\n" (or author email)))))
      ;; Date.
      (let ((date (and (plist-get info :with-date) (org-export-get-date info))))
-       (format "\\date{%s}\n" (org-export-data date info)))
+       (when date
+             (format "\\date{%s}\n" (org-export-data date info))))
      ;; Title and subtitle.
      (let* ((subtitle (plist-get info :subtitle))
 	    (formatted-subtitle