Bug #633942: Fix "nil" column header on single account report

When I open a report window (barchart in the example) and select only one
account, I get a column header saying "nil" near the only data column.

Someone accidentally wrote "nil" instead of the empty list '().

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@19783 57a11ea4-9604-0410-9ed3-97b8803252fd

src/report/standard-reports/category-barchart.scm

@@ -560,7 +560,7 @@ developing over time"))
                           all-data)
                      (if (> (gnc:html-table-num-columns table) 2)
                          (list (_ "Grand Total"))
-                         (list nil)
+                         '()
                          )
                      ))