Green rank_modules ranks unloadable as Manual

MSP-12557

Was calling `.class` blindly on the output of `create`, but `nil` has a
class, `NilClass`, so it didn't call `module_rank` as expected and
assigned NormaLRanking to `nil` instead of ManualRanking.

Author: limhoff-r7

lib/msf/core/module_set.rb

@@ -340,7 +340,11 @@ def module_rank(reference_name, metasploit_module_class)
       # TODO don't create an instance just to get the Class.
       created_metasploit_module_instance = create(reference_name)
 
-      module_rank(reference_name, created_metasploit_module_instance.class)
+      if created_metasploit_module_instance.nil?
+        module_rank(reference_name, nil)
+      else
+        module_rank(reference_name, created_metasploit_module_instance.class)
+      end
     elsif metasploit_module_class.const_defined? :Rank
       metasploit_module_class.const_get :Rank
     else