#59 Return "" instead of null

@drewnoakes 
We could also write the actual value instead of an empty string into the directory and use Charset.isSupported() in IptcReader to determine if TAG_CODED_CHARACTER_SET is a valid java encoding. Although I don't think this would be much of a benefit. Additionally it would require the correct handling of characters like 'ESC' so they don't appear as question marks.