Hide empty status texts

It seems like WhatsApp advertises an empty string if one doesn't set a
status text explicitly. This leads to strange formatting of buddies in
Pidgin's buddy list when buddy details are enabled. Prevent this by
pretending a NULL status text if the status message is empty.

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>
Merge pull request #38 from cryptocrack/hide-empty-status

Hide empty status texts