Comment # 16 on bug 953954 from Wolfgang Bauer

(In reply to Jaroslav Kamen�k from comment #15)
> kde.xembedsniproxy: trying to dock window 56623164
> kde.xembedsniproxy: adding damage watch for 56623164
> kde.xembedsniproxy: Resizing window 56623164 "Pidgin" from w*h 200 200
> kde.xembedsniproxy: Skip transparent xembed icon for 56623164 "Pidgin"
> kde.xembedsniproxy: Skip transparent xembed icon for 56623164 "Pidgin"
> kde.xembedsniproxy: Skip transparent xembed icon for 56623164 "Pidgin"
> kde.xembedsniproxy: Skip transparent xembed icon for 56623164 "Pidgin"

Well, this tells at least that Pidgin still uses XEmbed.
Enable the "Ubuntu Indicator" plugin (right-click on the blank system tray icon
and choose "Plugins") and it should use SNI instead and work.
You can then turn off the standard icon in the settings.

I did try to install pidgin now in a fresh Leap KDE VM, and I can reproduce
your issue.
Also pidgin works fine on my 13.2 system (with Plasma 5.4.3) though, so the
problem is definitely not related to xembed-sni-proxy, it rather looks like
pidgin does indeed paint a blank image for some reason.
Maybe something else is missing which causes this, on my 13.2 system I do have
a full GNOME installation alongside KDE.
It's strange though that pidgin's icon does show up in IceWM (in Leap)...