<data> section with empty <values> section causes error

[embeepea]

A <data> section that contains an empty <values> section (no actual values) causes the error

cannot read property 'length' of undefined

What it should do is to simply create a data object that contains no values and whose getIterator() method returns an empty iterator.

This came up while dealing with the timeline widget for the global climate dashboard. It isn't urgent --- I'm working around it by simply including a bogus value in the timeline's <data> section. But it should be fixed.