lualoader: Always return a proper dictionary for blacklist

If module_blacklist isn't specified, we have an empty blacklist; effectively
the same as if module_blacklist="" were specified in loader.conf(5).

This was reported when switching to a BE that predated the module_blacklist
introduction, but the problem is valid all the same and likely to be tripped
over in other scenarios.

Reported by: bwidawsk
MFC after: 3 days