Default value not shown for default slot when only named slot content provided

### Vue.js version
2.2.1

### Reproduction Link
https://jsfiddle.net/x4m8w2xv/1/

### Steps to reproduce
Having two slots, one named and the other is default, and template provides only other slot content, the default slot does not show default value because empty Nodes are registered with " " (empty space) as content.