Handle empty data array in completion collection

Previously, a response of { data: [] } from Backdrop would cause
this collection to crash the application.

var periods = response.data[0].values ? response.dat...
                              ^
TypeError: Cannot read property 'values' of undefined

This change ensures that the object is correctly structured before
attempting to access the length of the values array inside data.
Remove completion collection assignment: this.data

I don't think it's used anywhere.
Merge pull request #269 from alphagov/67207594-completion-handle-empty-array

Handle empty data in completion collection