context_page_build sets empty regions to empty arrays potentially confusing themes

The Context sets empty regions to empty arrays in context_page_build. This can confuse themes/modules that use isset to test for empty regions in hook_preprocess_html's $vars for example.

The patch below adjusts the code to only set the region if context has blocks to place in the region. (I'll attach the below as a patch file in the first comment).

     // Load all region content assigned via blocks.
     foreach (array_keys($all_regions) as $region) {
       if ($this->is_enabled_region($region)) {
-        $page[$region] = isset($page[$region]) ? array_merge($page[$region], $this->block_get_blocks_by_region($region)) : $this->block_get_blocks_by_region($region);
+        if ($blocks = $this->block_get_blocks_by_region($region)) {
+          $page[$region] = isset($page[$region]) ? array_merge($page[$region], $blocks) : $blocks;
+        }
       }
     }
   }