Do not interpret OlympusEq.LensType 0 0 0 0 0 0

[sarunasb]

Olympus/OMDS cameras use '0 0 0 0 0 0' LensType, when mechanical lense is used. Currently Exiv2 interprets this into localised string ‘None’. However Olympus cameras traditionally allow to enter a custom string for lens name and then add that to Exif LensModel of images taken. Image processing software can take None from the LensType and ignore LensModel. Having LensType of '0 0 0 0 0 0' would allow for an easy if statement and then use LensModel instead.