applet: Fix crash when opening context menu with no contents
Do not try to open the context menu via the right mouse button or the Menu key press when it has no contents, because that leads to a crash.
The menu button was already guarded against this because it gets hidden in that case.
BUG: 470847 FIXED-IN: 6
This fixes the crash and I think it is a helpful guard to be better safe than sorry (maybe even for Plasma5.27?).
The question now is why does the context menu has no contents on Plasma6.