(#12464) Avoid unnecessarily reloading facts when node_name_fact is not set

In fixing #8341 we reduced the caching of facts, leading to a performance
regression in puppet apply. Puppet apply loaded facts twice, first to look up
the fact for node_name_fact, and then a second time under the new node name
given by node_name_fact. This optimizes the common case where node_name_fact
is not set so that facts are only looked up once.

Reported in #12310.