return an empty excelx cell if a children are unknown

lib/roo/excelx/sheet_doc.rb

@@ -101,6 +101,8 @@ def cell_from_xml(cell_xml, hyperlink)
             return create_cell_from_value(value_type, cell, formula, format, style, hyperlink, base_date, coordinate)
           end
         end
+
+        Excelx::Cell::Empty.new(coordinate)
       end
 
       def create_cell_from_value(value_type, cell, formula, format, style, hyperlink, base_date, coordinate)

test/test_roo.rb

@@ -17,6 +17,8 @@
 # with the wrong spreadsheet class
 #STDERR.reopen "/dev/null","w"
 
+Encoding.default_external = "UTF-8"
+
 require 'test_helper'
 require 'stringio'