Don't crash on invalid exiv2 data

Summary:
The file from bug 375131 crashes `baloo_file_extractor`.
The problem is that its EXIF data contains a key `Exif.Photo.FocalLength`,
whose type is `Exiv2::unsignedRational`, and whose value is empty.
On the other hand, the `Exiv2::Value::toFloat()` call relies on at least single component of a value,
causing undefined behavior (i.e. crash) if there is none.

This is simple workaround: if we got a property with no value, just return an empty QVariant().
(unfortunately, didn't manage to reproduce the hang reported in the bug originally)

BUG: 352856
BUG: 353848
BUG: 361259
CCBUG: 375131

Test Plan: `baloo_file_extractor` no longer crashes on the file, it processes the file and extracts all the necessary data

Reviewers: #baloo, #frameworks, astippich

Reviewed By: astippich

Subscribers: bruns, astippich, kde-frameworks-devel

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D16165