Make sure logic less conditionals also check empty state.

In the documentation of the conditional it is stated that "If the object is not false or empty?, the content will show", but before this commit, we didn't check on `empty?`. Especially combined with the inverted conditional (where `empty?` is actually checked), this is very confusing, because this means a value can be true and false at the same time.

Author: tim-vandecasteele

lib/slim/logic_less/context.rb

@@ -27,7 +27,8 @@ def lambda(name)
       end
 
       def section(name)
-        if dict = scope[name]
+        dict = scope[name]
+        if dict && !(dict.respond_to?(:empty?) && dict.empty?)
           if !dict.respond_to?(:has_key?) && dict.respond_to?(:each)
             new_scope do
               dict.each do |d|

test/logic_less/test_logic_less.rb

@@ -302,6 +302,17 @@ def test_conditional_parent
     assert_html'<li class="previous"><a href="prev">Older</a></li><li class="next"><a href="next">Newer</a></li>', source, scope: {prev_page: 'prev', next_page: 'next'}
   end
 
+  def test_empty_conditional  
+    source = %q{
+- prev_page
+  li.previous
+    a href=prev_page Older
+- next_page
+  li.next
+    a href=next_page Newer}
+    assert_html'<li class="next"><a href="next">Newer</a></li>', source, scope: {prev_page: '', next_page: 'next'}
+  end
+
   def test_render_with_yield
     source = %q{
 div