String-type config options set to empty string shouldn't turn into None
    
    Without this, if we try to set a string-type config option to '' it becomes
    None.  When code expecting a string uses this value, it ends up expanding
    out to 'None' in various places, including the plugin configuration pages.
    
    Signed-off-by: Howard Johnson <merlin@merlinthp.org>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Merges: #97