[table view] allow to set None in 'headers', meaning the label should be fetched from the result set as done by default (closes #1931533) the point of this is to avoid having to define the whole headers list when only some of them have to be defined.

debian/tmp/usr/bin/cubicweb-ctl usr/bin/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/cwctl.py usr/lib/PY_VERSION/site-packages/cubicweb
debian/cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl