Encode empty values

The original library skips processing of empty cells, but this behavior
is not desirable for us. There is an [issue reported][1] about it and a
[PR fixing it][2], but the upstream author has not acted on it.

This is the reason we decided to fork the repository and patch it
manually. Credit goes to the patch's author for his work.

JRVS-306

[1]: felixbuenemann#43
[2]: paladinsoftware#2

lib/xlsxtream/row.rb

@@ -52,14 +52,12 @@ def to_xml
         else
           value = value.to_s
 
-          unless value.empty? # no xml output for for empty strings
-            value = value.encode(ENCODING) if value.encoding != ENCODING
-
-            if @sst
-              xml << %Q{<c r="#{cid}" t="s"><v>#{@sst[value]}</v></c>}
-            else
-              xml << %Q{<c r="#{cid}" t="inlineStr"><is><t>#{XML.escape_value(value)}</t></is></c>}
-            end
+          value = value.encode(ENCODING) if value.encoding != ENCODING
+
+          if @sst
+            xml << %Q{<c r="#{cid}" t="s"><v>#{@sst[value]}</v></c>}
+          else
+            xml << %Q{<c r="#{cid}" t="inlineStr"><is><t>#{XML.escape_value(value)}</t></is></c>}
           end
         end
       end

test/xlsxtream/row_test.rb

@@ -6,7 +6,7 @@ module Xlsxtream
   class RowTest < Minitest::Test
     def test_empty_column
       row = Row.new([nil], 1)
-      expected = '<row r="1"></row>'
+      expected = '<row r="1"><c r="A1" t="inlineStr"><is><t></t></is></c></row>'
       actual = row.to_xml
       assert_equal expected, actual
     end
@@ -148,7 +148,7 @@ def test_string_column_with_shared_string_table
 
     def test_multiple_columns
       row = Row.new(['foo', nil, 23], 1)
-      expected = '<row r="1"><c r="A1" t="inlineStr"><is><t>foo</t></is></c><c r="C1" t="n"><v>23</v></c></row>'
+      expected = '<row r="1"><c r="A1" t="inlineStr"><is><t>foo</t></is></c><c r="B1" t="inlineStr"><is><t></t></is></c><c r="C1" t="n"><v>23</v></c></row>'
       actual = row.to_xml
       assert_equal expected, actual
     end