[forms] Fix HTableFormRenderer _render_fields method behavior when display_label is False.
Indeed, now the extra <tr><th>...</th></tr> are not inserted anymore unless
display_label is True.
Closes #3811325.
usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py