cubicweb-keyword #2189225 get rid of deprecated .entity(0) calls [validation pending]

keyword still uses pre-cw 3.6 .entity(0) shortcut in a few places.

priorityimportant
typebug
done in1.7.0
load0.000
load left0.000
closed by#393fad2f3087 remove deprecated .entity(0) calls (closes #2189225)