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