tablerow: Avoid accidental special case for constant nil cols (#1644)

It should behave the same as an expression that evaluates to nil

lib/liquid/tags/table_row.rb

@@ -51,7 +51,7 @@ module Liquid
       collection = Utils.slice_collection(collection, from, to)
       length     = collection.length
 
-      cols = @attributes['cols'].nil? ? length : context.evaluate(@attributes['cols']).to_i
+      cols = @attributes.key?('cols') ? context.evaluate(@attributes['cols']).to_i : length
 
       output << "<tr class=\"row1\">\n"
       context.stack do

test/integration/tags/table_row_test.rb

@@ -66,6 +66,20 @@ class TableRowTest < Minitest::Test
       { 'characters' => '' })
   end
 
+  def test_cols_nil_constant_same_as_evaluated_nil_expression
+    expect = "<tr class=\"row1\">\n" \
+      "<td class=\"col1\">false</td>" \
+      "<td class=\"col2\">false</td>" \
+      "</tr>\n"
+
+    assert_template_result(expect,
+      "{% tablerow i in (1..2) cols:nil %}{{ tablerowloop.col_last }}{% endtablerow %}")
+
+    assert_template_result(expect,
+      "{% tablerow i in (1..2) cols:var %}{{ tablerowloop.col_last }}{% endtablerow %}",
+      { "var" => nil })
+  end
+
   def test_tablerow_loop_drop_attributes
     template = <<~LIQUID.chomp
       {% tablerow i in (1...2) %}