N3 parser rejects predicate-less statements allowed by the grammar

[jeswr]

The current N3 grammar makes predicateObjectList optional in triples, so the following are valid and emit no outer triple:

:s .
{:nested} .
[:p :o] .

N3.js currently errors when the terminating punctuation arrives without a predicate. This is exposed by cwm_syntax/zero-predicates.n3 in #640.

A small historical prototype exists as 82d6bbf, but it needs to be ported with current context handling and negative tests ensuring Turtle/TriG remain strict.

Part of #640.

[jeswr]

Already resolved on main by #641: _readPredicate permits punctuation with no predicate in N3 mode, and that PR includes bare-statement coverage. The W3C zero-predicates case is currently masked only by the undeclared empty prefix tracked in #687 / #693.