Return NaN if position info source doesn't have an attribute.

The previous returned value of -1 could be a valid value for some
attributes. Returning NaN has the advantage that calculations done with
an attribute that has not been set will have the final result evaluate
to NaN as well.

Change-Id: I42fd3512453910efd78f3be66acba46a1e829ee0
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>