handle nil in File.join

Honestly I don't know much about Fabricator, but when running pickle rspec, prefix is nil and makes it crash
this fixes it... thanks

lib/fabrication/schematic/manager.rb

@@ -45,7 +45,7 @@ def load_definitions
     preinitialize
     Fabrication::Config.path_prefixes.each do |prefix|
       Fabrication::Config.fabricator_paths.each do |folder|
-        Dir.glob(File.join(prefix, folder, '**', '*.rb')).sort.each do |file|
+        Dir.glob(File.join(prefix.to_s, folder, '**', '*.rb')).sort.each do |file|
           load file
         end
       end