Fix failing test of exiv gps data

Summary:
A zero value was used before to indicate invalid
data, and thus 0° latitude/longitude or 0 m altitude were
not displayed while being valid data. Use NaN to indicate
invalid data. Increase altitude precision by avoiding
intermediate float type

Reviewers: bruns

Reviewed By: bruns

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18010