[twitter] Don't crash if the username or password is unset
[twitter] Don't crash if the username or password is unset

ThomasBollmeier/libsocialweb-flickr-oauth,GNOME/libsocialweb,lcp/mojito,lcp/libsocialweb,ThomasBollmeier/libsocialweb-flickr-oauth,lcp/mojito,lcp/libsocialweb,lcp/libsocialweb,ThomasBollmeier/libsocialweb-flickr-oauth,GNOME/libsocialweb,lcp/mojito,GNOME/libsocialweb