Don't use search widget emptiness as a proxy for "no reusable references"

This works the first time the dialog is opened, but if the user types
a value into the search widget that yields 0 results, then closes
and reopens the dialog, that value will persist and the search widget
will still be empty, so we'll wrongly disable the "Use existing reference"
button.

Instead, check what we're actually interested in directly: whether
the index of reusable references is empty.

Bug: 61960
Change-Id: If4bf6ced7dc3af643f602c07f3142a0e637a3247
Merge "Don't use search widget emptiness as a proxy for "no reusable references""