Changed the 'nil' check to be an absence check

The previous version, was setting `all_selected` to be false if 
`$(this).attr('value') == ''`. However, ideally what we want is once the 
selected option's value is blank, we want to display the span.all_selected (which the current version doesn't do - but this change does).
Merge pull request #10 from marcamillion/patch-3

all_selected now shows in any blank state