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

stand/lua/config.lua

@@ -266,12 +266,12 @@ local function isValidComment(line)
 end
 
 local function getBlacklist()
+	local blacklist = {}
 	local blacklist_str = loader.getenv('module_blacklist')
 	if blacklist_str == nil then
-		return nil
+		return blacklist
 	end
 
-	local blacklist = {}
 	for mod in blacklist_str:gmatch("[;, ]?([%w-_]+)[;, ]?") do
 		blacklist[mod] = true
 	end