fix python 2.4 vs 2.5 syntax error

File "/usr/lib64/python2.4/site-packages/duplicity/commandline.py", line 188
    return encoding if encoding else 'utf-8'
                     ^
SyntaxError: invalid syntax 

see
http://lists.nongnu.org/archive/html/duplicity-talk/2012-10/msg00027.html