Empty strings treated as valid property values

[karger]

When a google spreadsheet import reports an empty string in a cell, that empty string is assigned as the value of the corresponding item property in exhibit. This causes tests like if-exists to return true. It seems more intuitive and consistent for an empty string to be treated as a null, so the corresponding property is taken to be not defined on the given item.

[white-gecko]

Alternatively it would be good to have some if-empty unfctionality in http://www.simile-widgets.org/wiki/Exhibit/Conditional_content

I've also tried it with:

<div ex:select=".key">
      <div ex:case="">not available</div>
      <div ex:content=".key"></div>
</div>

which also always ends up in the default case.