node,async-wrap: verify domain enter/exit are set

The REPL global object lazy loads modules by placing getters for each.
This causes MakeDomainCallback() to be run if a native module is loaded
from the REPL, but if the domain module hasn't been loaded then there
are no enter/exit callbacks to be called. Causing an assert() to fail.

Fix the issue by conditionally running the callback instead of asserting
it is available. Also add "addon" test to verify the fix.

Fixes: #8231
Signed-off-by: Trevor Norris <trev.norris@gmail.com>