Downgrade -man message of ignored empty paragraph to MANDOC_IGNPAR. The

Downgrade -man message of ignored empty paragraph to MANDOC_IGNPAR.  The
change in man_macro.c was from an assertion caused by a subtle problem:
(1) macro is removed, causing m->last to be m->last->parent; (2) by jumping
to the m->last->parent after post-validation, the original
m->last->parent is skipped; (3) the rewinder climbs to the root of the
tree and aborts.

The original issue recorded in the TODO by schwarze@, reminded by Brad
Smith.